crosshair: axis badges, clustered cursor labels, and the shared-y horizontal line

- Axis badges: the y value in the y label strip and the x time in the x strip, each a
  full-opacity panel in the crosshair's colour, **centred on the line it names**. The tick
  text is flushed before the fill -- the over-text batch is flushed after every line batch,
  so without that flush the label behind printed through the panel (the "transparent
  backplate" defect).
- Cursor labels: collected across the cursor loop and clustered at its end, so two marks in
  one column share a panel with one line each instead of overprinting, and a label stays on
  its own mark's row.
- Figure: a split row's cells share one y (`set_share_row_y`), so the horizontal line is
  drawn in every cell of the hovered row -- one value, one scale, spanning the row -- and
  still in no other row.

Tests: `figure_crosshair_horizontal_spans_a_split_rows_cells`, plus the render-smoke
additions for the badge placement/flush and the clustered labels.

include/plot/plot/axis.hpp

@@ -102,12 +102,16 @@ struct Axis {
   /// group stops following or the row leaves it.
   bool live_owner = false;
   f64 live_owner_newest = 0.0;
-  /// Set while this plot is a member of a `Figure` x link group (`Figure::link_x()`
+  /// Set while this axis is a member of a `Figure` link group. On an x axis that is
-  /// writes it, `unlink_row()` clears it). The group owns the shared x window *and*
+  /// an x link group (`Figure::link_x()` writes it, `unlink_row()` clears it): the
-  /// its follow state -- `Figure::link_x()` refuses a `Live` member joiner -- so
+  /// group owns the shared x window *and* its follow state -- `Figure::link_x()`
-  /// `Plot::set_x_follow(Live)` on this axis is refused after linking too, rather
+  /// refuses a `Live` member joiner -- so `Plot::set_x_follow(Live)` on this axis is
-  /// than letting a later call put the member's axis back into a state the group
+  /// refused after linking too, rather than letting a later call put the member's
-  /// cannot drive (see `Plot::set_x_follow()`).
+  /// axis back into a state the group cannot drive (see `Plot::set_x_follow()`).
+  /// On a y axis it is the row's shared-y group (`Figure::set_share_row_y()`): the
+  /// group, not the plot, owns the range *and* the auto scale state, so
+  /// `Plot::autoscale_axes()` skips the axis and the figure fits the union of the
+  /// members' traces instead.
   bool link_member = false;
   /// Ticks for the last `refresh()`, ascending, majors and (when asked) minors.
   std::vector<Tick> ticks;

include/plot/plot/crosshair.hpp

@@ -91,6 +91,14 @@ struct Crosshair {
   /// On by default, and free to leave on: a plot with no visible trace has no rows to
   /// measure and the panel is then not placed at all.
   bool show_readout = true;
+  /// Write the two values the crosshair's lines meet the axes at as small badges in the
+  /// label strips -- the y value beside the y ticks, the x value beside the x ticks, each
+  /// in its axis' own format. On by default: it is the axis reading an oscilloscope puts
+  /// where the eye already is, and a host that does not want it turns it off
+  /// (`Plot::set_crosshair_axis_labels()`). The badges are placed only when the strip they
+  /// belong to exists and the line they belong to is drawn (a vertical-only crosshair has
+  /// no y to badge, and a row whose x strip went to a neighbour badges nothing).
+  bool axis_labels = true;
   /// The data x of the vertical line, on x axis 0.
   f64 x = 0.0;
   /// The data y of the horizontal line, on y axis 0. Meaningless for a vertical-only
@@ -277,6 +285,17 @@ struct CrosshairLayout {
   /// `Crosshair::show_readout` is off, or when the panel could not be placed inside the
   /// axes at all.
   Rect panel{};
+  /// The two axis badges, when `Crosshair::axis_labels` is on and the crosshair is active:
+  /// the y value in the **y label strip**, right-aligned on the axis line at the horizontal
+  /// line's height, and the x value in the **x label strip**, centred on the vertical
+  /// line's column -- each in its axis' own format, so a badge reads exactly like the tick
+  /// labels beside it. Zero-sized (and the text empty) when the flag is off, the crosshair
+  /// is inactive, the line they belong to is not drawn, their strip does not exist (a row
+  /// whose x strip went to the row below), or there is no text backend to measure them.
+  Rect y_axis_badge{};
+  std::string y_axis_text;
+  Rect x_axis_badge{};
+  std::string x_axis_text;
   /// One row per visible trace, in trace order -- see `CrosshairRow`. Present whether or
   /// not the panel could list them, so the markers are drawn even in a plot too small to
   /// hold a panel.

include/plot/plot/figure.hpp

@@ -32,15 +32,16 @@ struct FigureLayout {
   /// rect here; a row of several tiles its rect left to right with `col_gap` between
   /// cells, the cells' widths summing to the row's less the gaps.
   std::vector<Rect> cells;
-  /// The strip the figure's one shared x axis is drawn in: the bottom row's own
+  /// The strip the figure's bottom x axis is drawn in: the bottom row's own
   /// `PlotLayout::x_label_area` (for a bottom row split into several cells, only the **last
   /// cell's** strip -- the others' strips are their own `plot(i).layout().x_label_area`, which
-  /// is where the controller's axis-zoom hit test reads each cell's gestures from), at the bottom of the bottom row's rect and so as
+  /// is where the controller's axis-zoom hit test reads each cell's gestures from), at the
-  /// wide as that row's axes rather than the whole figure. Read back from the row
+  /// bottom of the bottom row's rect and so as wide as that row's axes rather than the whole
+  /// figure. Read back from the row
   /// rather than derived here, so a host and the frame cannot disagree about where
   /// the axis is. It is nonzero whenever the figure has rows and the bottom row
-  /// reserves a label strip -- including the case where every row labels itself, in
+  /// reserves a label strip. A row above whose columns differ from the row below keeps
-  /// which case it is simply the bottom row's own strip, one of several.
+  /// a strip of its own (see `Figure`), which is its plots' `x_label_area`, not this.
   ///
   /// It is the rect the controller's axis-zoom hit test reads (`x_label_area` is what
   /// a middle drag in a label strip enters `DragMode::AxisZoomX` from), which is why
@@ -70,12 +71,15 @@ struct FigureLayout {
 /// **The shared x axis.** Row heights are an equal division of the figure by
 /// default, `set_row_weights()` gives a row more of it, and every row keeps a floor
 /// height so chrome can never erase a plot. When `set_x_labels_only_on_bottom()` is
-/// on -- the default -- the figure's one x axis is the **bottom row's own**: the rows
+/// on -- the default -- the x axis is the **bottom row's own** for every run of rows
-/// above reserve no x chrome at all, and the bottom row keeps its own label and title
+/// with the *same columns* (the same number of cells and the same normalised cell
-/// strips, so those labels and that title are drawn once, from the bottom row's own
+/// weights): those rows above reserve no x chrome at all, and the bottom row of the run
-/// ticks and transform. That places the shared axis *inside* a row, where the
+/// keeps its own label and title strips, so those labels and that title are drawn once,
-/// controller's axis-zoom hit test can reach it, instead of in a band outside every
+/// from its own ticks and transform. A row whose columns differ from the row below keeps
-/// row that no gesture could land in.
+/// its own strip, drawn under each of its cells, because the axis below is at another
+/// scale and width. The rule is structural, never a function of what a plot holds. That
+/// places a shared axis *inside* a row, where the controller's axis-zoom hit test can
+/// reach it, instead of in a band outside every row that no gesture could land in.
 ///
 /// **The x link.** `plot(i).set_x_range()` and a controller gesture both reach a
 /// member's x range the same way, so a figure cannot see either of them happen.
@@ -186,14 +190,17 @@ class Figure {
   /// than row by row for the left gutter's reason: the rows' axes have to line up, and
   /// they end on a common column only if they are told one width. See
   /// `Plot::set_right_gutter()` for what it is for. Negative values are clamped to 0.
+  /// In a split row whose cells share one y (`set_share_row_y`) only the **last** cell
+  /// keeps it, taken off the row like the y gutter, so the gap between two plots of the
+  /// row is the column gap.
   void set_right_gutter(f32 device_px) noexcept {
     right_gutter_ = device_px > 0.0f ? device_px : 0.0f;
   }
   f32 right_gutter() const noexcept { return right_gutter_; }
 
-  /// Whether the shared x axis is the figure's: every row reserves no x label strip
+  /// Whether rows with the same columns as the row below hand it their x axis (the
-  /// and one shared axis is drawn under the last row (the default), or every row
+  /// default; a row with different columns keeps its own), or every row draws its own
-  /// draws its own labels and the figure draws nothing extra.
+  /// labels and the figure draws nothing extra.
   void set_x_labels_only_on_bottom(bool on) noexcept { labels_bottom_only_ = on; }
   bool x_labels_only_on_bottom() const noexcept { return labels_bottom_only_; }
 
@@ -240,9 +247,12 @@ class Figure {
 
   /// Places the crosshair with the pointer's **plot** and that plot's local y (spec §17): the
   /// shared x goes to every plot -- vertical lines everywhere, as the plain overload does -- and
-  /// the horizontal line is drawn **only in `hovered_row`** -- the **flattened plot index**, not
+  /// the horizontal line is drawn in `hovered_row` -- the **flattened plot index**, not the
-  /// the figure-row index; they differ when a row holds several cells -- where its y is a value of
+  /// figure-row index; they differ when a row holds several cells -- where its y is a value of
-  /// *that* plot's own scale. It is the fix for the reported defect -- a figure showing no horizontal
+  /// *that* plot's own scale. **When the row is a split whose cells share one y
+  /// (`set_share_row_y`), the line is drawn in every cell of that row**: it is one value on one
+  /// scale, so stopping it at the divider would hide the number the neighbouring cell is read
+  /// against. Rows never share y with each other, so no other row gets a line. It is the fix for the reported defect -- a figure showing no horizontal
   /// line at all -- without inventing a figure-wide y, which would mean a different value in
   /// every row: the line appears where the pointer is, and no row ever shows another row's y.
   ///
@@ -314,6 +324,22 @@ class Figure {
   /// A group's follow state. `Off` for group 0 and for a group that does not exist.
   XFollow x_link_follow(u32 group) const noexcept;
 
+  /// Makes the cells of every split row (two or more) share their y axis 0: one range
+  /// and one auto scale state per row, default off. A range set on any member -- a
+  /// wheel, a drag, `Plot::set_y_range()` -- becomes the row's and is pushed to all of
+  /// its cells at the next `layout()`; the row is auto or manual as one, and while it is
+  /// auto it fits the union of every cell's visible traces (a cell with none is ignored;
+  /// padding and log handling are the plot's own, via `Plot::fit_y_bounds()`). The group,
+  /// not the plot, owns auto scale: members' own autoscale is suppressed, because each
+  /// would fit only its own traces and the cells would fight over the range. Turning it
+  /// on or off forgets the groups' state; a row of one cell is never a group.
+  void set_share_row_y(bool on);
+  bool share_row_y() const noexcept { return share_row_y_; }
+  /// Whether figure row `row` is auto scaled as a group. True for a row that is not a
+  /// group -- fewer than two cells, sharing off, or a row that does not exist -- so
+  /// "auto" is also the answer when there is no shared state to be manual.
+  bool row_y_auto(u32 row) const noexcept;
+
  private:
   /// One link group: the range its members share, the scroll state behind that
   /// range while it follows, the rows in it (ascending) and, in step with them, the
@@ -370,6 +396,21 @@ class Figure {
   /// a following group's window, push both to all members.
   void sync_x_links();
 
+  /// A split row's shared y axis: the range last pushed to its cells (the value the
+  /// next frame compares each cell against, so a gesture is told from the group's own
+  /// write -- the same device as `XLink::pushed`), whether the group has been
+  /// initialised from its first cell, and whether it is auto scaled.
+  struct RowY {
+    Range pushed{};
+    bool have = false;
+    bool auto_scale = true;
+  };
+  /// Advances every split row's y group for this frame: adopt what a cell was changed
+  /// to (a range or an auto flag), fit the union of the cells' traces while the group
+  /// is auto, push the result to all cells. Runs before any `Plot::layout()`, which is
+  /// where a plot would otherwise autoscale on its own.
+  void sync_row_y();
+
   /// Fills the row bookkeeping (`cells_per_row_`, `row_first_`, `cell_weights_`) from a
   /// per-row cell count, clamping each to at least one. Does not touch `plots_`, which the
   /// constructors size from `total_cells()`.
@@ -381,6 +422,8 @@ class Figure {
   /// index of row r's first cell and the last entry the plot count.
   std::vector<u32> row_first_;
   std::vector<std::vector<f32>> cell_weights_;
+  bool share_row_y_ = false;
+  std::vector<RowY> row_y_;
   f32 col_gap_ = 0.0f;
   /// What `plot()` yields for a row that does not exist, so a stale row can be used
   /// without reading out of bounds. Owned rather than a static, so one figure's

include/plot/plot/plot.hpp

@@ -40,6 +40,13 @@ namespace plot {
 /// are for a host that wants one without the other.
 enum class XStrip { Both, LabelsOnly, TitleOnly, None };
 
+/// Which parts of the left y chrome `Plot::layout()` reserves. `Both` is a plot drawn on
+/// its own. `None` reserves no label width, no title column and ignores the figure's common
+/// gutter: it is the mode of every cell of a split row but the first when the figure shares
+/// the row's y axis (`Figure::set_share_row_y`), where the first cell draws the one axis.
+/// The cell's ticks still refresh and its grid still builds.
+enum class YStrip { Both, None };
+
 /// Whether a strip mode reserves the label strip. One rule, read by `Plot::layout()`
 /// when it sizes the strip and by the renderer when it decides whether to stage the
 /// labels -- a row that reserves none would otherwise stage them over its own data.
@@ -238,6 +245,12 @@ class Plot {
   void set_x_strip_mode(XStrip mode) noexcept { x_strip_ = mode; }
   XStrip x_strip_mode() const noexcept { return x_strip_; }
 
+  /// Which parts of the left y chrome `layout()` reserves; `YStrip::Both` by default. With
+  /// `YStrip::None` the y label width, the y title column and the figure's common gutter are
+  /// all zero, and the renderer stages no y labels or title (`y_label_area.w == 0`).
+  void set_y_strip_mode(YStrip mode) noexcept { y_strip_ = mode; }
+  YStrip y_strip_mode() const noexcept { return y_strip_; }
+
   /// The font layout() measures tick labels with. Borrowed, not owned, and may be
   /// null -- then the label areas fall back to PlotLayout's defaults and no string
   /// is measured: a plot with no renderer lays out with whatever the host sets
@@ -435,6 +448,20 @@ class Plot {
   /// does not exist. Not for callers outside the figure: a `Plot` that is not in a
   /// figure never has a linked axis.
   void set_x_link_member(u32 axis, bool linked) noexcept;
+  /// The y-side twin of `set_x_link_member()`: publishes that this y axis is a member
+  /// of its row's shared-y group (`Figure::set_share_row_y()`). A linked y axis is
+  /// never fitted by `layout()` -- the group owns its range and auto scale, and fits
+  /// the union of every member's traces -- but a gesture or `set_y_range()` still
+  /// writes it, which is how the group learns of a change. A no-op for an axis that
+  /// does not exist. Not for callers outside the figure.
+  void set_y_link_member(u32 axis, bool linked) noexcept;
+  /// Union of the bounds of the visible traces bound to y axis `axis`: what a shared-y
+  /// group collects from each member. False -- and `out` untouched -- when none.
+  bool y_data_bounds(u32 axis, Range& out) const;
+  /// Fits y axis `axis` to `bounds` the way `auto_fit_y()` fits it to its own traces
+  /// (same padding, same log handling), leaving `auto_scale` alone. False -- range
+  /// untouched -- when the axis does not exist or cannot be fitted to `bounds`.
+  bool fit_y_bounds(u32 axis, Range bounds);
 
   /// Data -> pixels for one axis, built from the axis' range, log flag and the
   /// current layout. Always invertible: a collapsed range or a collapsed axes
@@ -530,6 +557,14 @@ class Plot {
   /// belongs to. An id that names no cursor is ignored, like every other setter here.
   void set_cursor_color(u32 id, Color color);
 
+  /// A cursor's caption (see `Cursor::label`): a vertical marker's badge text -- a horizontal
+  /// ruler writes its own value and needs no caption to be readable. Rendered on a
+  /// backplate, and labels whose backplates meet are merged into one panel with one line per
+  /// label (see the renderer), which is what keeps a marker pair half a division apart
+  /// readable instead of a smear of two texts over one column. An empty string removes the
+  /// caption; an id that names no cursor is ignored, like every other setter here.
+  void set_cursor_label(u32 id, std::string label);
+
   /// A cursor's stroke: its logical width (`<= 0` for the style's default) and its dash
   /// (`dash.off <= 0` for solid). The instrument's own, like its colour -- see `Cursor::width`.
   void set_cursor_stroke(u32 id, f32 width, Vec2 dash);
@@ -861,6 +896,14 @@ class Plot {
   /// **not** part of `set_crosshair()`, so placing a hover cannot silently change it.
   void set_crosshair_readout(bool on) noexcept { crosshair_.show_readout = on; }
 
+  /// Writes or clears the two **axis badges** (see `Crosshair::axis_labels`): the y value in
+  /// the y label strip and the x value in the x label strip, at the heights and columns the
+  /// crosshair's lines meet the axes at. Takes effect on the next `layout()`. **Not** part of
+  /// `set_crosshair()`, for the same reason the readout flag is not.
+  void set_crosshair_axis_labels(bool on) noexcept { crosshair_.axis_labels = on; }
+  /// The axis-badge flag, as set.
+  bool crosshair_axis_labels() const noexcept { return crosshair_.axis_labels; }
+
   /// Overrides this plot's crosshair stroke and dash, in logical pixels, without touching the
   /// global style every other plot draws with (spec §17): each field left negative in `s`
   /// defers to the matching `Style::crosshair_*` metric, so `CrosshairStyle{}` (the default)
@@ -1060,6 +1103,8 @@ class Plot {
   std::string x_note_;
   /// Which parts of the bottom x chrome are reserved at all; see set_x_strip_mode().
   XStrip x_strip_ = XStrip::Both;
+  /// Which parts of the left y chrome are reserved at all; see set_y_strip_mode().
+  YStrip y_strip_ = YStrip::Both;
   /// The floor a figure imposes on the left gutter; see set_min_y_gutter(). 0.0 means
   /// none, which is what a plot standing on its own keeps.
   f32 min_y_gutter_ = 0.0f;

include/plot/render/renderer.hpp

@@ -95,6 +95,24 @@ struct FrameStats {
 /// a guard -- and the frame reports it (see `last_frame_had_gl_error()`).
 ///
 /// Not thread-safe, and confined to the thread that owns the current context.
+
+/// One cursor label's measured text and placed badge: what `Renderer::collect_cursor_label()`
+/// builds and `Renderer::draw_clustered_cursor_labels()` places and draws. A rendering
+/// detail of the cursor labels' backplates, declared at namespace scope only because a
+/// private nested type cannot be named in an out-of-class member definition's signature.
+struct CursorLabel {
+  std::string text;  ///< the one line the backplate carries
+  Color border{};    ///< the panel's border: the cursor's own colour
+  f32 anchor = 0.0f; ///< the coordinate the label belongs to (x for a marker, y for a ruler)
+  f32 w = 0.0f;      ///< the measured text width
+  bool horizontal = false;
+  /// The placed pen and backplate: centred on the anchor across the mark, nudged onto a
+  /// free row when a close neighbour's badge is in the way, and clustered into one panel
+  /// by `draw_clustered_cursor_labels()`.
+  Vec2 pen{};
+  Rect box{};
+};
+
 class Renderer {
  public:
   Renderer() noexcept = default;
@@ -238,12 +256,13 @@ class Renderer {
   /// exactly as a stand-alone plot is, and `draw(Plot&)` itself is unchanged for
   /// single-plot hosts.
 
-  /// The figure's one shared x axis is the **bottom row's own**, so it is drawn by
+  /// A shared x axis is the **bottom row's own** (of a run of rows with the same
-  /// that row's normal path: when the figure says labels belong on the bottom only
+  /// columns), so it is drawn by that row's normal path: when the figure says labels
-  /// (the default), the rows above reserve no x chrome and the bottom row stages its
+  /// belong on the bottom only (the default), the rows above that match the row below
-  /// own labels and title once, inside its own strip -- the rect
+  /// reserve no x chrome and the bottom row stages its own labels and title once,
-  /// `FigureLayout::x_axis_strip` reads back. With that switched off, every row keeps
+  /// inside its own strip -- the rect `FigureLayout::x_axis_strip` reads back. A row
-  /// its own chrome and each draws its own labels; there is then no shared axis.
+  /// whose columns differ keeps and draws its own; with that switched off, every row
+  /// does and there is no shared axis.
 
   /// Styling, the text backend and `ui_scale` are installed on **every** row before
   /// the figure computes its layout, so the strips the rows measure and the strokes
@@ -620,13 +639,22 @@ class Renderer {
   /// the clip, and a range maximum that undershoots the edge by a ulp is not mistaken for
   /// off the plot.
   void draw_cursors(const Plot& plot, const PlotLayout& layout);
-  /// One cursor's label: the caller's caption for either orientation, and for a horizontal
+  /// One cursor's label, measured rather than drawn: a horizontal ruler's line is the value
-  /// ruler the value itself, written in its y axis' own tick format (`Axis::format_value()`)
+  /// itself, written in its y axis' own tick format (`Axis::format_value()`) -- the number a
-  /// -- the number a ruler exists to be read off. A vertical cursor's caption starts at the
+  /// ruler exists to be read off -- with the caption first on the line when there is one; a
-  /// top of the axes beside its marker; a ruler's text starts at the axes' left edge on its
+  /// vertical cursor's line is its caption, beside the marker at the top of the axes. The
-  /// own row, with the caption first on that line when there is one.
+  /// label is then placed (centred on its mark, stacked clear of close neighbours) and drawn
-  void draw_cursor_label(const Plot& plot, const PlotLayout& layout, const Cursor& cursor,
+  /// clustered (`draw_clustered_cursor_labels()`), which is what keeps a pair of readout
-                         f32 at);
+  /// marks half a division apart readable: two texts over one column is a smear, one panel
+  /// with two lines is a readout.
+  void collect_cursor_label(const Plot& plot, const PlotLayout& layout, const Cursor& cursor,
+                            f32 at, std::vector<CursorLabel>& labels);
+  /// Draws the placed labels' backplates -- one panel per cluster of meeting boxes, every
+  /// line at its own placed pen -- and stages the lines. Clustering is what keeps a pair of
+  /// readout marks half a division apart readable: two texts over one column is a smear, one
+  /// panel with two lines is a readout.
+  void draw_clustered_cursor_labels(const PlotLayout& layout,
+                                    const std::vector<CursorLabel>& labels);
   /// The regions: the fill, one bound per edge, the optional label. Every rect comes
   /// from `PlotLayout::regions` -- the same geometry the controller hit-tests -- so
   /// a region is drawn where it can be grabbed, and neither side derives the other's
@@ -655,8 +683,12 @@ class Renderer {
   /// outermost pixels the fill covers (the crisp-hairline rule -- a border drawn on the measured
   /// rect's fractional edges is lit half-heartedly, and a frame bright on three sides and dim on the
   /// fourth reads as a mistake). One helper, two callers: the border's colour is the only thing a
-  /// legend panel and a readout panel do not share.
+  /// legend panel and a readout panel do not share. `background_alpha` scales the frame's own
-  void draw_readout_panel(const Rect& panel, Color border);
+  /// background under the border: the legend and the crosshair's readout keep the shared
+  /// translucent backplate (what they cover stays faintly visible), while a *badge* -- a
+  /// cursor's value box, the crosshair's axis badges -- is read against whatever it sits on
+  /// (ticks, a noisy trace) and is drawn opaque.
+  void draw_readout_panel(const Rect& panel, Color border, float background_alpha);
   /// One trace's colour swatch in `box`, opaque whatever the trace's own alpha is: a swatch drawn at
   /// a trace's 15% would be the least legible thing in the frame, and the swatch is the one element
   /// that says which colour an entry or a readout row *is*. The legend's entries and the crosshair's
@@ -686,6 +718,12 @@ class Renderer {
   /// Drawn after every other interior pass -- the legend included -- because it is what the
   /// pointer is on: a readout the legend covered would be the feature failing at its one job.
   void draw_crosshair_readout(const Plot& plot, const PlotLayout& layout);
+  /// The crosshair's **axis badges** (`CrosshairLayout::y_axis_badge`/`x_axis_badge`): the
+  /// values the two lines meet the axes at, each on the shared readout backplate with its
+  /// border in the crosshair's colour, drawn over the tick labels they sit among. Called
+  /// from `draw_plot()` after the label strips are drawn, which is the only order in which a
+  /// badge in a strip can sit on the ticks and still be read.
+  void draw_crosshair_axis_badges(const PlotLayout& layout);
 
   /// The host's box-zoom frame (`PlotLayout::selection`): a fill at `selection_fill` under a border at
   /// `selection_edge`, or nothing at all when nothing was swept. Drawn last of the interior passes, so

src/plot/figure.cpp

@@ -73,6 +73,38 @@ f32 place_cells(const Rect& r, u32 n, const std::vector<f32>& weights, f32 col_g
   return gap;
 }
 
+/// The share of the row each of its `n` cells is given by `weights`, exactly as `place_cells`
+/// reads them: an empty (or wrongly sized, or non-positive-total) vector means equal shares.
+std::vector<f32> normalised_shares(const std::vector<f32>& weights, u32 n) {
+  std::vector<f32> shares(n, 1.0f / static_cast<f32>(n));
+  f32 total = 0.0f;
+  if (weights.size() == n) {
+    for (const f32 w : weights) total += w;
+  }
+  if (total > 0.0f) {
+    for (u32 i = 0; i < n; ++i) shares[i] = weights[i] / total;
+  }
+  return shares;
+}
+
+/// Whether two rows have the **same columns**: the same number of cells and the same
+/// normalised cell weights (within 1e-4). The rows are all the figure's width, so such rows
+/// get cells with identical x extents -- the same y gutter, gap, floors and right gutter
+/// apply to both -- and one row's x axis serves the other. Structural, never content: it
+/// reads the cell counts and the weights the host set and nothing a plot holds.
+bool same_columns(const std::vector<u32>& cells_per_row,
+                  const std::vector<std::vector<f32>>& cell_weights, u32 row_a, u32 row_b) {
+  const u32 n = cells_per_row[row_a];
+  if (n != cells_per_row[row_b]) return false;
+  if (n == 1u) return true;
+  const std::vector<f32> a = normalised_shares(cell_weights[row_a], n);
+  const std::vector<f32> b = normalised_shares(cell_weights[row_b], n);
+  for (u32 i = 0; i < n; ++i) {
+    if (std::fabs(a[i] - b[i]) > 1e-4f) return false;
+  }
+  return true;
+}
+
 /// A coordinate in the shared axis' **axis space**: `log10` when the figure's one
 /// x axis -- the bottom row's, the row the shared strip belongs to -- is
 /// logarithmic, the value itself otherwise. A following group's span and offset
@@ -434,6 +466,81 @@ bool Figure::adopt_member_changes(XLink& group) {
   return false;
 }
 
+void Figure::set_share_row_y(bool on) {
+  share_row_y_ = on;
+  row_y_.clear();
+  for (u32 row = 0; row < cells_per_row_.size(); ++row) {
+    if (cells_per_row_[row] < 2u) continue;
+    for (u32 i = row_first_[row]; i < row_first_[row] + cells_per_row_[row]; ++i) {
+      plots_[i].set_y_link_member(0u, on);
+    }
+  }
+}
+
+bool Figure::row_y_auto(u32 row) const noexcept {
+  return row < row_y_.size() && row_y_[row].have ? row_y_[row].auto_scale : true;
+}
+
+void Figure::sync_row_y() {
+  if (!share_row_y_) return;
+  row_y_.resize(cells_per_row_.size());
+  for (u32 row = 0; row < cells_per_row_.size(); ++row) {
+    const u32 n = cells_per_row_[row];
+    if (n < 2u) continue;
+    RowY& g = row_y_[row];
+    const u32 first = row_first_[row];
+    // Axes added after set_share_row_y() are covered here.
+    for (u32 i = first; i < first + n; ++i) plots_[i].set_y_link_member(0u, true);
+    if (!g.have) {
+      g.pushed = plots_[first].y_axis(0u).range;
+      g.auto_scale = plots_[first].y_axis(0u).auto_scale;
+    }
+    // What happened since the last push. A member's auto flag that differs from the group's
+    // is a request (set_y_range clears it, auto_fit_y / set_y_auto_scale(true) sets it);
+    // otherwise a member whose range differs from the pushed one was moved by a gesture.
+    bool want_auto = g.auto_scale;
+    Range target = g.pushed;
+    bool flag_request = false;
+    for (u32 i = first; i < first + n && !flag_request; ++i) {
+      const Axis& a = plots_[i].y_axis(0u);
+      if (a.auto_scale != g.auto_scale) {
+        want_auto = a.auto_scale;
+        if (!want_auto) target = a.range;  // auto -> manual: the gesture's range
+        flag_request = true;
+      }
+    }
+    if (!flag_request && !want_auto) {
+      for (u32 i = first; i < first + n; ++i) {
+        const Range r = plots_[i].y_axis(0u).range;
+        if (r.min != g.pushed.min || r.max != g.pushed.max) {
+          target = r;
+          break;
+        }
+      }
+    }
+    if (want_auto) {
+      Range bounds{};
+      bool any = false;
+      for (u32 i = first; i < first + n; ++i) {
+        Range b{};
+        if (!plots_[i].y_data_bounds(0u, b)) continue;
+        bounds = any ? Range{std::min(bounds.min, b.min), std::max(bounds.max, b.max)} : b;
+        any = true;
+      }
+      // The first cell stands in for the row: all cells are assumed to share one y axis kind/padding (the host builds them identically).
+      if (any && plots_[first].fit_y_bounds(0u, bounds)) target = plots_[first].y_axis(0u).range;
+      // else: nothing to fit -- keep the range the group had
+    }
+    for (u32 i = first; i < first + n; ++i) {
+      plots_[i].set_y_range(0u, target);          // clears the flag ...
+      plots_[i].set_y_auto_scale(0u, want_auto);  // ... so restate it: every member mirrors the group
+    }
+    g.pushed = target;
+    g.auto_scale = want_auto;
+    g.have = true;
+  }
+}
+
 void Figure::sync_x_links() {
   for (XLink& group : links_) {
     if (group.rows.empty()) continue;
@@ -483,12 +590,13 @@ const FigureLayout& Figure::layout() {
   }
 
   const f32 figure_h = std::max(0.0f, viewport_.h);
-  // The rows tile the figure itself: the shared x axis is the *bottom row's own*, so
+  // The rows tile the figure itself: an x axis is a row's *own* strip (the bottom row's, and
-  // nothing is taken off the figure for it. That is what keeps the axis reachable --
+  // the strip of any row whose columns differ from the row below), so nothing is taken off
-  // its strip is a real `PlotLayout::x_label_area` inside the bottom row's rect, which
+  // the figure for it. That is what keeps the axis reachable -- its strip is a real
-  // is the rect the controller's axis-zoom hit test reads and the strip an x title is
+  // `PlotLayout::x_label_area` inside a row's rect, which is the rect the controller's
-  // drawn in -- and it is why an interior row reserves no x chrome at all instead of
+  // axis-zoom hit test reads and the strip an x title is drawn in -- and it is why a row
-  // leaving a band no one owns.
+  // that hands its axis to the one below reserves no x chrome at all instead of leaving a
+  // band no one owns.
   const f32 content_h = figure_h;
   // The gap is dropped, rather than squeezing the rows, when the figure cannot hold
   // it: a figure one pixel tall with four rows still gives every row a positive
@@ -558,29 +666,48 @@ const FigureLayout& Figure::layout() {
     if (n > 1u && used < col_gap_) layout_.col_gap = 0.0f;
   }
 
-  const u32 bottom_row = rows - 1u;
+  // Which rows carry their own x strip, decided structurally and before the first
+  // measurement pass so that pass and the final one see the same strips. The bottom row
+  // always has one. A row above hides its strip only when the row directly below has the
+  // same columns (see `same_columns()`), because then the cells of the two rows have the
+  // same x extents and the axis below serves this row too; the rule is transitive, since
+  // that row is itself either labelled or hidden in favour of one that matches it. A row
+  // whose columns differ from the one below -- `[1][2]` over `[3]` -- keeps its own strip,
+  // drawn under each of its cells, because the axis below is at another scale and width.
+  // A figure told to label every row keeps the strips everywhere.
+  std::vector<XStrip> strips(rows, XStrip::Both);
+  if (labels_bottom_only_) {
+    for (u32 row = rows - 1u; row-- > 0u;) {
+      // By induction the row below is labelled or itself hidden in favour of a matching
+      // row, so hiding this one on a match with it is the transitive rule.
+      if (same_columns(cells_per_row_, cell_weights_, row, row + 1u)) strips[row] = XStrip::None;
+    }
+  }
   for (u32 row = 0; row < rows; ++row) {
-    // One x axis for the whole figure, and it is the bottom row's: every plot keeps its
+    const XStrip strip = strips[row];
-    // x chrome -- labels, title and the strip the controller's axis zoom is found in --
+    // A split row whose cells share one y range also shares one y axis, drawn by its first
-    // and every plot above the bottom row gives all of it to that row, so the labels and
+    // cell: every other cell reserves no y chrome at all (see `YStrip::None`).
-    // the title are not drawn once per row. A figure told to label every row keeps them
+    const bool shared_y = share_row_y_ && cells_per_row_[row] > 1u;
-    // everywhere, and then there is no shared axis to speak of: each plot has its own. A
+    // ... and its right gutter is the last cell's alone, so the gap between two plots of the
-    // split bottom row labels each of its cells, each against its own x axis.
+    // row is the column gap and not a gutter more (see the placement block below).
-    const XStrip strip = labels_bottom_only_ && row != bottom_row ? XStrip::None : XStrip::Both;
+    const u32 last = row_first_[row + 1u] - 1u;
     for (u32 i = row_first_[row]; i < row_first_[row + 1u]; ++i) {
       Plot& p = plots_[i];
       p.set_viewport(layout_.cells[i]);
       p.set_x_strip_mode(strip);
+      p.set_y_strip_mode(shared_y && i != row_first_[row] ? YStrip::None : YStrip::Both);
       // Before either pass below, and the same value for every plot: nothing is measured on
       // the right, so unlike the left gutter this needs no measurement pass -- the plots
       // end on a common column because they are all told the one width.
-      p.set_right_gutter(right_gutter_);
+      p.set_right_gutter(shared_y && i != last ? 0.0f : right_gutter_);
     }
   }
   // After the viewports, so a row's ticks and grid -- built from its axes rect -- and
   // its transforms describe this frame's rects; and before the rows lay themselves
   // out below, so a following group's window is the range they lay out with.
   sync_x_links();
+  // Before any `Plot::layout()`, where a plot would autoscale its own y axis.
+  sync_row_y();
   // Every row is laid out against its own rect, so after this call each row's
   // chrome is current: `plot(i).layout()` reads back what the figure computed rather
   // than what the row last held. That is the contract `Renderer::draw(Figure&)`
@@ -611,25 +738,59 @@ const FigureLayout& Figure::layout() {
   // Over the flattened plots: every cell of every row takes part, so the y axes of a split
   // row line up with each other and with the rows above and below.
   f32 gutter = min_y_gutter_;
+  //
+  // A `YStrip::None` cell draws no y axis, so it has no width to contribute and takes no part
+  // in the maximum: the gutter is the widest of the cells that do draw one.
   for (Plot& p : plots_) {
     p.set_min_y_gutter(0.0f);
     const PlotLayout& row_layout = p.layout();
+    if (p.y_strip_mode() == YStrip::None) continue;
     gutter = std::max(gutter, row_layout.y_axis_width + row_layout.y_title_width);
   }
+  // Rows that share one y axis divide their *plot* widths, not their cell widths: the first
+  // cell's y strip would otherwise make its plot narrower than its neighbours'. The gutter is
+  // taken off the row first, the remainder is split by `place_cells` (so the weights, the gap
+  // and the floor all apply to plot widths), and the first cell then grows left by the gutter
+  // to hold the axis. The gutter is capped at half the row, so a row too narrow for it
+  // degrades to ordinary cells instead of producing a negative rect. The cells still tile the
+  // row: the first starts on its left edge and the last still ends on its right edge.
+  //
+  // The right gutter is taken off the same way and belongs to the **last** cell alone (the
+  // other cells were told none above): left to every cell, the visible gap between two
+  // plots would be the column gap plus a whole right gutter. Together with the y gutter it is
+  // capped at half the row, and never exceeds the figure's own right gutter.
+  for (u32 row = 0; row < rows; ++row) {
+    const u32 n = cells_per_row_[row];
+    if (!share_row_y_ || n <= 1u) continue;
+    const Rect& r = layout_.rows[row];
+    const f32 g_eff = std::max(0.0f, std::min(gutter, r.w / 2.0f));
+    const f32 rg_eff = std::max(0.0f, std::min(right_gutter_, r.w / 2.0f - g_eff));
+    Rect* cells = &layout_.cells[row_first_[row]];
+    const f32 used = place_cells(Rect{r.x + g_eff, r.y, r.w - g_eff - rg_eff, r.h}, n,
+                                 cell_weights_[row], col_gap_, floor_scale, cells);
+    cells[0].x -= g_eff;
+    cells[0].w += g_eff;
+    cells[n - 1u].w += rg_eff;
+    layout_.col_gap = used < col_gap_ ? 0.0f : layout_.col_gap;
+    for (u32 i = row_first_[row]; i < row_first_[row + 1u]; ++i) plots_[i].set_viewport(layout_.cells[i]);
+    // What the cell actually reserved, so a row too narrow for the whole gutter is not asked
+    // for more than it was given (the same value whenever the row has room).
+    plots_[row_first_[row + 1u] - 1u].set_right_gutter(rg_eff);
+  }
   // A one-row figure imposes exactly what that row measured, so it lays out as the plot
   // would on its own; a figure whose rows all need the same strip imposes that, which is
   // also what each would have done alone.
   for (Plot& p : plots_) {
-    p.set_min_y_gutter(gutter);
+    p.set_min_y_gutter(p.y_strip_mode() == YStrip::None ? 0.0f : gutter);
     p.layout();
   }
 
-  // The figure's one x axis is the bottom row's, so the strip it is drawn in is that
+  // `x_axis_strip` is the bottom row's own label area (the last plot's) -- the rect the
-  // row's own label area -- the rect the controller hit-tests and the renderer stages
+  // controller hit-tests and the renderer stages its labels in, read back here rather than
-  // its labels in, read back here rather than derived again, so a host and the frame
+  // derived again, so a host and the frame cannot disagree about where it is. Rows above
-  // cannot disagree about where the shared axis is. It is zero-sized only when there
+  // that carry their own strip (their columns differ from the row below, or every row is
-  // are no rows at all: with `set_x_labels_only_on_bottom(false)` it is simply the last
+  // labelled) have it in their plots' own `layout().x_label_area`. It is zero-sized only
-  // row's own strip, one of several.
+  // when there are no rows at all.
   layout_.x_axis_strip = plots_.back().layout().x_label_area;
   return layout_;
 }
@@ -658,6 +819,21 @@ void Figure::set_crosshair(f64 x, u32 hovered_row, f64 y_in_row, bool active) {
   // than clearing a hover the pointer still has.
   if (!active || hovered_row >= plots_.size()) return;
   plots_[hovered_row].set_crosshair(x, y_in_row, true);
+  // **The cells of a split row share one y**, so the horizontal line means the same value in
+  // every one of them: it spans the row rather than stopping at the divider between two cells
+  // that read the same scale. Only that row -- rows do not share y with each other, so a
+  // sibling row's line would be a value on a scale it does not have -- and only when the row
+  // really is split (`set_share_row_y`, which is what makes the y common in the first place).
+  if (!share_row_y_) return;
+  const u32 row = row_of_plot(hovered_row);
+  if (row >= cells_per_row_.size() || cells_per_row_[row] <= 1u) return;
+  const u32 first = row_first_[row];
+  for (u32 k = 0; k < cells_per_row_[row]; ++k) {
+    const u32 cell = first + k;
+    if (cell != hovered_row && cell < plots_.size()) {
+      plots_[cell].set_crosshair(x, y_in_row, true);
+    }
+  }
 }
 
 void Figure::clear_crosshair() {

src/plot/plot.cpp

@@ -311,6 +311,11 @@ void Plot::set_cursor_color(u32 id, Color color) {
   cursors_[id].color = color;
 }
 
+void Plot::set_cursor_label(u32 id, std::string label) {
+  if (id >= cursors_.size()) return;
+  cursors_[id].label = std::move(label);
+}
+
 void Plot::set_cursor_stroke(u32 id, f32 width, Vec2 dash) {
   if (id >= cursors_.size()) return;
   cursors_[id].width = width;
@@ -819,6 +824,19 @@ void Plot::set_x_link_member(u32 axis, bool linked) noexcept {
   x_axes_[axis].link_member = linked;
 }
 
+void Plot::set_y_link_member(u32 axis, bool linked) noexcept {
+  if (axis >= y_axes_.size()) return;
+  y_axes_[axis].link_member = linked;
+}
+
+bool Plot::y_data_bounds(u32 axis, Range& out) const {
+  return trace_bounds_union(/*x_side=*/false, axis, out);
+}
+
+bool Plot::fit_y_bounds(u32 axis, Range bounds) {
+  return axis < y_axes_.size() && fit_axis_range(y_axes_[axis], bounds);
+}
+
 XFollow Plot::x_follow(u32 axis) const {
   return axis < x_axes_.size() ? x_axes_[axis].follow : XFollow::Off;
 }
@@ -1001,7 +1019,8 @@ void Plot::autoscale_axes() {
     if (x_axes_[i].auto_scale && !follows_live(i)) fit_x_axis(i);
   }
   for (u32 i = 0; i < y_axes_.size(); ++i) {
-    if (y_axes_[i].auto_scale) fit_y_axis(i);
+    // A shared-y member is fitted by its figure, against every cell's traces.
+    if (y_axes_[i].auto_scale && !y_axes_[i].link_member) fit_y_axis(i);
   }
 }
 
@@ -1298,13 +1317,18 @@ const PlotLayout& Plot::layout() {
     y_width = y_axes_.empty() ? 0.0f : 2.0f * pad;
     x_height = text_->height() + 2.0f * pad;
   }
-  // A row of a `Figure` gives its x strip to the figure's one shared axis -- the
+  // A row of a `Figure` that hands its x axis to the row below (which has the same
-  // bottom row's own strip, where that axis lives -- so it reserves none: the seed and
+  // columns, so the same x extents) reserves no strip: the seed and
   // the fixed point below both start from zero, and `axes` then reaches the viewport's
   // bottom edge. Set here rather than in the loop, because the loop's `need_x` is what
   // the strip wants and this overrides the want, not the measurement.
   const bool x_labels = x_strip_has_labels(x_strip_);
   if (!x_labels) x_height = 0.0f;
+  // The y strip likewise: a cell that gives its y axis to a neighbour (`YStrip::None`) reserves
+  // no label width, no title column and no share of the figure's gutter, so every y-strip
+  // quantity below collapses to zero and `axes` starts at the viewport's left edge.
+  const bool y_none = y_strip_ == YStrip::None;
+  if (y_none) y_width = 0.0f;
   // The title strips, though, are needed before the loop and never change, unlike
   // the label strips: a strip holds exactly one text line per titled axis and no
   // tick label can change that. Nothing at all is reserved without a text backend
@@ -1318,7 +1342,7 @@ const PlotLayout& Plot::layout() {
   // no title reserves no row for a title either, and one that reserves the title and no
   // labels still gets its row -- below where the labels would have been.
   const u32 x_titles = x_strip_has_title(x_strip_) ? titled_x_axis_count() : 0u;
-  const f32 y_title_w = text_ != nullptr ? y_title_strip_width() : 0.0f;
+  const f32 y_title_w = text_ != nullptr && !y_none ? y_title_strip_width() : 0.0f;
   const f32 x_title_h = text_ != nullptr && x_titles != 0u
                             ? static_cast<f32>(x_titles) * text_->height() + 2.0f * pad
                             : 0.0f;
@@ -1339,7 +1363,7 @@ const PlotLayout& Plot::layout() {
     // -- and it is applied before the title column below, so the pair still fits the cap
     // (the floor is clamped to the same budget) and the invariant `axes.w >= share` holds.
     // `min_y_gutter_` is 0.0 unless a figure set it, so this is a no-op for a lone plot.
-    const f32 y_floor = std::min(min_y_gutter_, y_budget);
+    const f32 y_floor = y_none ? 0.0f : std::min(min_y_gutter_, y_budget);
     y_width = std::min(std::max(y_width, y_floor - y_title_w), y_budget);
     x_height = std::min(x_height, x_budget);
     const f32 y_title_width = std::min(y_title_w, y_budget - y_width);
@@ -1372,7 +1396,8 @@ const PlotLayout& Plot::layout() {
     if (text_ == nullptr || pass + 1 >= kMaxLayoutPasses) break;
     // The same caps as above, so this compares like with like and the loop still
     // reaches its fixed point on the second pass.
-    const f32 need_y = std::min(std::clamp(y_label_width(), 0.0f, vw), y_budget);
+    const f32 need_y =
+        y_none ? 0.0f : std::min(std::clamp(y_label_width(), 0.0f, vw), y_budget);
     // Overridden, not merely unmeasured: the strip is not reserved, so the fixed
     // point has to be zero for a row that gave its strip away.
     const f32 need_x =
@@ -1868,6 +1893,14 @@ void Plot::build_crosshair() {
   const auto inside = [](f32 v, f32 lo, f32 hi) {
     return std::clamp(v, std::min(lo, hi), std::max(lo, hi));
   };
+  crosshair.y_axis_badge = Rect{};
+  crosshair.y_axis_text.clear();
+  crosshair.x_axis_badge = Rect{};
+  crosshair.x_axis_text.clear();
+  if (!crosshair_.active) {
+    crosshair.rows.clear();
+    return;
+  }
 
   // The two lines, each a stroke box held *inside* the axes by half its own width. The
   // renderer stages the stroke across the box's centre line, so this is the whole placement
@@ -1898,6 +1931,58 @@ void Plot::build_crosshair() {
     crosshair.horizontal = Rect{axes.x, at - half, axes.w, stroke};
   }
 
+  // The axis badges, when asked for (`Crosshair::axis_labels`): the values the two lines
+  // meet the axes at, written in the strips the tick labels are written in and in each
+  // axis' own format, so a badge reads exactly like the tick beside it. Placed here rather
+  // than by the renderer, for the readout panel's reason -- one placement rule, one place to
+  // read it -- and left empty whenever there is nothing honest to place: no text backend to
+  // measure with, the line they belong to not drawn (a vertical-only crosshair has no y),
+  // or their strip absent or too small (a row whose x strip went to the row below, a y
+  // gutter collapsed to a title). The x badge is held to the axes' own columns as well, so
+  // a crosshair in the leftmost cell of a split row cannot badge into its neighbour's.
+  if (crosshair_.axis_labels && text_ != nullptr) {
+    const f32 badge_pad = style_.device(4.0f);
+    const f32 badge_h = text_->height() + 2.0f * badge_pad;
+    if (crosshair_.horizontal && layout_.y_label_area.w > 0.0f &&
+        layout_.y_label_area.h >= badge_h) {
+      char text[64];
+      y_axis(0u).format_value(crosshair_.y, text, sizeof text);
+      crosshair.y_axis_text.assign(text);
+      const f32 w = text_->measure(crosshair.y_axis_text.c_str()) + 2.0f * badge_pad;
+      // Right-aligned on the axis line -- where the y ticks end -- and **centred** on the
+      // horizontal line's own height, the way the x badge is centred on its column: the badge
+      // names the line it sits beside, and a badge whose top edge *is* that line reads as
+      // belonging to the tick below it. A badge is as wide as its own text and is capped by
+      // nothing else, so it
+      // is held within the **left strips' union**: the label strip first, the title strip
+      // beside it when there is one, and never over the data or off the viewport's left
+      // edge. A `std::clamp` on an inverted pair is undefined, which is exactly the state
+      // "the strip is narrower than the badge" produces -- hence the ordered min/max chain.
+      const f32 left = layout_.y_title_area.w > 0.0f ? layout_.y_title_area.x
+                                                     : layout_.y_label_area.x;
+      f32 bx = layout_.y_label_area.right() - w;
+      if (bx < left) bx = left;
+      bx = std::min(bx, axes.x - w);
+      if (bx < left) bx = left;  // a badge wider than the union stays at its left edge
+      const f32 at = inside(static_cast<f32>(ty.to_screen_y(crosshair_.y)) - 0.5f * badge_h,
+                            layout_.y_label_area.y + badge_pad,
+                            layout_.y_label_area.bottom() - badge_pad - badge_h);
+      crosshair.y_axis_badge = Rect{bx, at, w, badge_h};
+    }
+    if (crosshair_.vertical && layout_.x_label_area.w > 0.0f &&
+        layout_.x_label_area.h >= badge_h) {
+      char text[64];
+      x_axis(0u).format_value(crosshair_.x, text, sizeof text);
+      crosshair.x_axis_text.assign(text);
+      const f32 w = text_->measure(crosshair.x_axis_text.c_str()) + 2.0f * badge_pad;
+      const f32 column =
+          inside(static_cast<f32>(tx.to_screen_x(crosshair_.x)), axes.x, axes.right());
+      const f32 x = inside(column - 0.5f * w, layout_.x_label_area.x,
+                           layout_.x_label_area.right() - w);
+      crosshair.x_axis_badge = Rect{x, layout_.x_label_area.y, w, badge_h};
+    }
+  }
+
   // The rows: one per *visible* trace -- a trace with no series or no samples is not under
   // the pointer, so it has nothing to say -- named or not, because the readout is about what
   // the pointer is over and not about identity. The vector is filled in place and its entries

src/render/renderer.cpp

@@ -480,10 +480,11 @@ void Renderer::draw(Figure& figure) {
   // than laying each row out a second time. Nothing draws before this: a row's
   // viewport is the figure's to set.
   //
-  // The figure's one x axis needs no pass of its own: it is the bottom row's own, so
+  // A shared x axis needs no pass of its own: it is the bottom row's own (of a run of rows
-  // that row stages it in its own strip through `draw_tick_labels()` like any other
+  // with the same columns), so that row stages it in its own strip through
-  // plot's axis -- and every row above it reserved no x chrome, which is what keeps
+  // `draw_tick_labels()` like any other plot's axis -- and every row of the run above it
-  // the labels from appearing once per row without a second staging here.
+  // reserved no x chrome, which is what keeps the labels from appearing once per row without
+  // a second staging here. A row whose columns differ stages its own.
   figure.layout();
   for (u32 row = 0; row < rows; ++row) {
     Plot& plot = figure.plot(row);
@@ -564,6 +565,10 @@ void Renderer::draw_plot(Plot& plot, const PlotLayout& layout) {
   clear_clip();
   draw_tick_labels(plot, layout);
   draw_titles(plot, layout);
+  // The crosshair's axis badges, last of all: they sit in the label strips among the tick
+  // labels, so they are drawn after them and unclipped -- and over them, which is the point
+  // (a badge the ticks print through is two numbers in one place).
+  draw_crosshair_axis_badges(layout);
 }
 
 void Renderer::end_frame() noexcept {
@@ -878,8 +883,8 @@ void Renderer::draw_tick_labels(const Plot& plot, const PlotLayout& layout) {
   const f32 line_h = font_backend_.height();
 
   // X labels: unless the plot gave its x label strip away -- a row of a `Figure`
-  // that reserves no x chrome, because the figure draws one shared axis in the bottom
+  // that reserves no x chrome, because the row below has the same columns and draws the
-  // row's own strip instead -- in which case it has no strip to put them in and their
+  // axis in its own strip instead -- in which case it has no strip to put them in and their
   // absence is the point. A plot with no strip would otherwise stage its labels just
   // above `axes`' bottom edge, i.e. over its own data. Centred under their tick on the
   // strip's single text line, which is the one rule for a plot's labels and a figure's
@@ -902,9 +907,14 @@ void Renderer::draw_tick_labels(const Plot& plot, const PlotLayout& layout) {
   // Y labels: right-aligned in their strip, centred on the tick and pulled back
   // inside the strip at the two extremes (a label centred on the top tick would
   // otherwise hang above the plot area).
+  //
+  // Unless the plot gave its y strip away (`YStrip::None`, a later cell of a split row that
+  // shares one y axis): its label area is then zero wide, and labels right-aligned against it
+  // would be drawn in the neighbouring cell. Keyed on the area rather than the mode so a strip
+  // the layout clamped to nothing stages nothing either.
   const f32 top_limit =
       std::max(layout.y_label_area.y, layout.y_label_area.bottom() - line_h);
-  for (u32 i = 0; i < plot.y_axis_count(); ++i) {
+  for (u32 i = 0; layout.y_label_area.w > 0.0f && i < plot.y_axis_count(); ++i) {
     const Transform transform = plot.y_transform(i);
     for (const Tick& tick : plot.y_axis(i).ticks) {
       if (!tick.major || tick.label.empty()) continue;
@@ -1669,6 +1679,9 @@ void Renderer::draw_cursors(const Plot& plot, const PlotLayout& layout) {
   // does not depend on the marker's own colour differing from the accent -- a cursor
   // created in the accent colour is otherwise indistinguishable from an idle one.
   const HotHandles hot = plot.hot_handles();
+  // Collected across the loop below and drawn clustered at its end -- see
+  // `draw_clustered_cursor_labels()` for why one panel can carry several lines.
+  std::vector<CursorLabel> labels;
 
   for (u32 i = 0; i < plot.cursor_count(); ++i) {
     const Cursor& cursor = plot.cursor(i);
@@ -1724,34 +1737,143 @@ void Renderer::draw_cursors(const Plot& plot, const PlotLayout& layout) {
       flush_lines(mapping(plot, cursor.axis_x, 0, x_drawn),
                   on ? style_.cursor : cursor.color, width, cursor.dash);
     }
-    draw_cursor_label(plot, layout, cursor, drawn);
+    // The labels are drawn after the strokes, clustered: collecting them all first is what
+    // lets two marks in the same column share one panel instead of overprinting each other.
+    collect_cursor_label(plot, layout, cursor, drawn, labels);
   }
+  draw_clustered_cursor_labels(layout, labels);
 }
 
-void Renderer::draw_cursor_label(const Plot& plot, const PlotLayout& layout, const Cursor& cursor,
+void Renderer::collect_cursor_label(const Plot& plot, const PlotLayout& layout,
-                                 f32 at) {
+                                    const Cursor& cursor, f32 at,
-  const f32 gap = style_.device(3.0f);
+                                    std::vector<CursorLabel>& labels) {
+  (void)layout;
   if (cursor.is_horizontal()) {
     // A ruler's text is the value it is *for*, written the way the y axis writes its own
     // tick labels, so a value read off a ruler reads exactly like the tick beside it. The
-    // caller's caption, when there is one, is staged first on the same line, so neither the
+    // caller's caption, when there is one, leads the same line.
-    // caption nor the value has to be dropped -- and both stay one text run's worth of
-    // glyphs, since the pen advances per glyph either way.
-    const Vec2 pen{layout.axes.x + gap, at + style_.device(2.0f)};
     char value[32];
     plot.y_axis(cursor.axis_y).format_value(cursor.y, value, sizeof value);
-    if (cursor.label.empty()) {
+    const std::string text =
-      stage_text(glyphs_under_, pen, value, style_.axis_text);
+        cursor.label.empty() ? std::string(value) : cursor.label + " " + value;
-      return;
+    CursorLabel label;
-    }
+    label.text = text;
-    stage_text(glyphs_under_, pen, cursor.label.c_str(), style_.axis_text);
+    label.border = cursor.color;
-    stage_text(glyphs_under_, {pen.x + font_backend_.measure(cursor.label.c_str()) + gap, pen.y},
+    label.anchor = at;  // the ruler's own row: the badge is centred on it
-               value, style_.axis_text);
+    label.w = font_backend_.measure(text.c_str());
+    label.horizontal = true;
+    labels.push_back(std::move(label));
     return;
   }
-  if (!cursor.label.empty()) {
+  if (cursor.label.empty()) return;
-    stage_text(glyphs_under_, {at + gap, layout.axes.y + style_.device(2.0f)},
+  CursorLabel label;
-               cursor.label.c_str(), style_.axis_text);
+  label.text = cursor.label;
+  label.border = cursor.color;
+  label.anchor = at;  // the marker's own column: the badge is centred on it
+  label.w = font_backend_.measure(cursor.label.c_str());
+  labels.push_back(std::move(label));
+}
+
+void Renderer::draw_clustered_cursor_labels(const PlotLayout& layout,
+                                            const std::vector<CursorLabel>& labels) {
+  if (labels.empty()) return;
+  const f32 pad = style_.device(3.0f);
+  const f32 line_h = font_backend_.height();
+  const Rect& axes = layout.axes;
+
+  // **Placement first, clustering second.** Each badge is centred on the mark it belongs to
+  // -- a ruler's badge straddles the ruler's own row, a marker's badge straddles the
+  // marker's own column -- because a badge that drifts from its mark is a number without an
+  // anchor. When two badges meet, the later one drops one text line (downward from its
+  // centred position) until it is clear, so a column of close rulers reads as a stack of
+  // numbers in their own rows rather than as texts printed through each other; the rows keep
+  // each badge beside its own mark, which a sequential stack from the panel's top would not.
+  std::vector<CursorLabel> placed;
+  placed.reserve(labels.size());
+  std::vector<CursorLabel> by_anchor = labels;
+  std::stable_sort(by_anchor.begin(), by_anchor.end(),
+                   [](const CursorLabel& a, const CursorLabel& b) { return a.anchor < b.anchor; });
+  for (CursorLabel& label : by_anchor) {
+    f32 x = 0.0f;
+    f32 y = 0.0f;
+    if (label.horizontal) {
+      x = axes.x + pad;
+      y = label.anchor - 0.5f * line_h;  // centred on the ruler's own row
+    } else {
+      // Centred on the marker's column, held inside the axes: a badge half over an edge is
+      // half a badge.
+      x = std::clamp(label.anchor - 0.5f * label.w, axes.x + pad,
+                     std::max(axes.x + pad, axes.right() - pad - label.w));
+      y = axes.y + pad;
+    }
+    for (;;) {
+      const Rect box{x - pad, y - pad, label.w + 2.0f * pad, line_h + 2.0f * pad};
+      bool meets = false;
+      for (const CursorLabel& p : placed) {
+        meets = box.x < p.box.x + p.box.w && p.box.x < box.x + box.w &&
+                box.y < p.box.y + p.box.h && p.box.y < box.y + box.h;
+        if (meets) break;
+      }
+      label.pen = Vec2{x, y};
+      label.box = box;
+      if (!meets) break;
+      y += line_h;  // one row down: still beside the mark, no longer on its neighbour
+    }
+    placed.push_back(label);
+  }
+
+  // The panels: one per cluster of meeting backplates -- the boxes' union plus padding, the
+  // border the cluster's first label's own -- with every line staged at its *placed* pen, so
+  // the panel is a unified plate behind badges that each still sit on their own mark.
+  const f32 margin = style_.device(2.0f);
+  std::vector<Rect> bounds;
+  std::vector<std::vector<std::size_t>> groups;
+  std::vector<char> used(placed.size(), 0);
+  for (std::size_t i = 0; i < placed.size(); ++i) {
+    if (used[i] != 0) continue;
+    used[i] = 1;
+    bounds.push_back(placed[i].box);
+    groups.emplace_back(1, i);
+    Rect& u = bounds.back();
+    for (bool grew = true; grew;) {
+      grew = false;
+      for (std::size_t j = 0; j < placed.size(); ++j) {
+        if (used[j] != 0) continue;
+        const Rect inflated{u.x - margin, u.y - margin, u.w + 2.0f * margin,
+                            u.h + 2.0f * margin};
+        const Rect& b = placed[j].box;
+        const bool meet = inflated.x < b.x + b.w && b.x < inflated.x + inflated.w &&
+                          inflated.y < b.y + b.h && b.y < inflated.y + inflated.h;
+        if (!meet) continue;
+        used[j] = 1;
+        groups.back().push_back(j);
+        const f32 rx = std::min(u.x, b.x);
+        const f32 ry = std::min(u.y, b.y);
+        u = Rect{rx, ry, std::max(u.x + u.w, b.x + b.w) - rx,
+                 std::max(u.y + u.h, b.y + b.h) - ry};
+        grew = true;
+      }
+    }
+  }
+  for (std::size_t g = 0; g < groups.size(); ++g) {
+    Rect panel = bounds[g];
+    panel.x -= pad;
+    panel.y -= pad;
+    panel.w += 2.0f * pad;
+    panel.h += 2.0f * pad;
+    // Inside the axes, shifted rather than clipped: a panel that hangs over an edge is a
+    // second plot; a panel nudged along it is still a readout of what it labels. Opaque: a
+    // badge is read against ticks and traces, and the badge's line beneath it is covered --
+    // the box interrupting its own ruler is what makes the two read as one instrument.
+    panel.x = std::clamp(panel.x, axes.x,
+                         std::max(axes.x, axes.right() - panel.w));
+    panel.y = std::clamp(panel.y, axes.y,
+                         std::max(axes.y, axes.bottom() - panel.h));
+    draw_readout_panel(panel, placed[groups[g].front()].border, 1.0f);
+    for (std::size_t k = 0; k < groups[g].size(); ++k) {
+      const CursorLabel& label = placed[groups[g][k]];
+      stage_text(glyphs_under_, label.pen, label.text.c_str(), style_.axis_text);
+    }
   }
 }
 
@@ -2017,7 +2139,7 @@ void Renderer::draw_legend(const Plot& plot, const PlotLayout& layout) {
   // The panel first, so every entry is on top of it. A legend's border is the frame's axis colour;
   // the readout's is the crosshair's own (see draw_crosshair_readout), which is the only thing that
   // differs between the two panels -- so it is the one parameter the shared helper takes.
-  if (plot.legend().frame) draw_readout_panel(legend.panel, style_.axis_line);
+  if (plot.legend().frame) draw_readout_panel(legend.panel, style_.axis_line, kLegendPanelAlpha);
 
   for (const LegendRow& row : legend.rows) {
     // The truncation row names no trace: it has no swatch to colour and its text is
@@ -2047,7 +2169,7 @@ void Renderer::draw_x_note(const Plot& plot, const PlotLayout& layout) {
   // A sliver of axes gets no caption rather than one hanging over the frame: the panel is chrome
   // *inside* the axes, and a backplate crossing the axis line would read as a second plot.
   if (panel.w > axes.w || panel.h > axes.h) return;
-  draw_readout_panel(panel, style_.axis_line);
+  draw_readout_panel(panel, style_.axis_line, kLegendPanelAlpha);
   // Bottom-left, beside the labels it qualifies -- the reader adds the caption to a tick, so it
   // belongs at the end of the tick row and not in a corner of its own. `glyphs_under_` because
   // the panel is flushed with the frame's line geometry and the under-text batch goes out after
@@ -2116,12 +2238,13 @@ void Renderer::draw_selection_frame(const PlotLayout& layout) {
   flush_lines(pixel_mapping(), style_.selection_edge, style_.device(1.0f));
 }
 
-void Renderer::draw_readout_panel(const Rect& panel, Color border) {
+void Renderer::draw_readout_panel(const Rect& panel, Color border, float background_alpha) {
   if (!(panel.w > 0.0f) || !(panel.h > 0.0f)) return;
-  // The frame's own background at a partial alpha, which dims whatever it covers instead of erasing
+  // The frame's own background scaled by the caller's opacity, which dims whatever it covers
-  // it -- the one backplate both readout panels use.
+  // -- or, at full opacity, covers it outright: a badge read against ticks or a noisy trace
+  // is read *through* nothing.
   Color background = style_.background;
-  background.a *= kLegendPanelAlpha;
+  background.a *= background_alpha;
   fill_pixels(panel, background);
   // The border frames the pixels the fill covers, not the measured rect's own fractional edges:
   // those are wherever the content happened to measure to, and a hairline drawn on one is lit
@@ -2220,14 +2343,47 @@ void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout)
   }
 }
 
+void Renderer::draw_crosshair_axis_badges(const PlotLayout& layout) {
+  const CrosshairLayout& crosshair = layout.crosshair;
+  if (!crosshair.active) return;
+  const bool y_badge = crosshair.y_axis_badge.w > 0.0f && !crosshair.y_axis_text.empty();
+  const bool x_badge = crosshair.x_axis_badge.w > 0.0f && !crosshair.x_axis_text.empty();
+  if (!y_badge && !x_badge) return;
+  // The same backplate the readout panel has, its border in the crosshair's colour too --
+  // the badge is that panel's value column, moved to where the line meets the axis it
+  // names. Drawn here rather than in the readout pass because it sits in a *label strip*:
+  // the strips are drawn after the interior clip is cleared, so a badge that lived in the
+  // clipped pass would be trimmed to nothing.
+  //
+  // **The tick labels are flushed first, and that is the whole point of this panel.** They
+  // are staged into the over-text batch, which the frame flushes after *every* line batch --
+  // so a fill drawn straight away would sit under them and the label it covers would print
+  // through the badge (the "transparent" backplate). They are already placed at this point,
+  // so pushing them out costs one batch: the fill then covers them, and the badge's own text
+  // is staged after both fills and leaves with the rest of the over batch, over everything.
+  flush_text(glyphs_over_);
+  const f32 pad = style_.device(4.0f);
+  if (y_badge) draw_readout_panel(crosshair.y_axis_badge, style_.crosshair, 1.0f);
+  if (x_badge) draw_readout_panel(crosshair.x_axis_badge, style_.crosshair, 1.0f);
+  if (y_badge) {
+    stage_text(glyphs_over_,
+               {crosshair.y_axis_badge.x + pad, crosshair.y_axis_badge.y + pad},
+               crosshair.y_axis_text.c_str(), style_.axis_text);
+  }
+  if (x_badge) {
+    stage_text(glyphs_over_,
+               {crosshair.x_axis_badge.x + pad, crosshair.x_axis_badge.y + pad},
+               crosshair.x_axis_text.c_str(), style_.axis_text);
+  }
+}
+
 void Renderer::draw_crosshair_readout(const Plot& plot, const PlotLayout& layout) {
   const CrosshairLayout& crosshair = layout.crosshair;
   if (crosshair.empty()) return;
-
   // The readout: the same backplate a legend has (the shared `draw_readout_panel()`), with its
   // border in the crosshair's own colour -- which is what makes the panel read as part of the
   // instrument that placed it rather than as a second legend.
-  draw_readout_panel(crosshair.panel, style_.crosshair);
+  draw_readout_panel(crosshair.panel, style_.crosshair, kLegendPanelAlpha);
 
   for (const CrosshairRow& row : crosshair.rows) {
     if (!row.listed) continue;

tests/test_figure.cpp

@@ -1257,10 +1257,11 @@ PLOT_TEST(figure_cells_share_x_link_and_crosshair) {
 }
 
 PLOT_TEST(figure_cells_x_labels_bottom_row_only) {
-  plot::Figure figure = split_figure({1, 2});
+  // Rows with the same columns share the bottom one's axis: {1,1,1} labels the last row only.
+  plot::Figure figure = split_figure({1, 1, 1});
   const plot::FigureLayout& fl = figure.layout();
   PLOT_CHECK(figure.plot(0u).layout().x_label_area.h == 0.0f);
-  PLOT_CHECK(figure.plot(1u).layout().x_label_area.h > 0.0f);
+  PLOT_CHECK(figure.plot(1u).layout().x_label_area.h == 0.0f);
   PLOT_CHECK(figure.plot(2u).layout().x_label_area.h > 0.0f);
   PLOT_CHECK_NEAR(fl.x_axis_strip.h, figure.plot(2u).layout().x_label_area.h, 0.001);
 }
@@ -1275,3 +1276,421 @@ PLOT_TEST(figure_cells_common_y_gutter) {
   PLOT_CHECK(g0 > 0.0);
   PLOT_CHECK_NEAR(g0, g1, 0.01);
 }
+
+PLOT_TEST(figure_row_y_cells_share_a_range_after_a_gesture) {
+  plot::Figure figure = split_figure({2, 1});
+  figure.set_share_row_y(true);
+  figure.layout();
+  figure.plot(1u).set_y_range(0u, {-3.0, 7.0});  // what a wheel / drag / pan ends in
+  figure.layout();
+  PLOT_CHECK_NEAR(figure.plot(0u).y_range(0u).min, -3.0, 1e-9);
+  PLOT_CHECK_NEAR(figure.plot(0u).y_range(0u).max, 7.0, 1e-9);
+  PLOT_CHECK_NEAR(figure.plot(1u).y_range(0u).min, -3.0, 1e-9);
+  PLOT_CHECK_NEAR(figure.plot(1u).y_range(0u).max, 7.0, 1e-9);
+  // The unsplit row is its own thing.
+  PLOT_CHECK_NEAR(figure.plot(2u).y_range(0u).min, -1.0, 1e-9);
+  PLOT_CHECK(!figure.row_y_auto(0u));
+}
+
+/// The horizontal line is one *value*, and a split row's cells share one y scale: the line
+/// spans the whole row instead of stopping at the divider between two cells that read the same
+/// numbers. The rows of the figure still do not share y, so no other row gets one.
+PLOT_TEST(figure_crosshair_horizontal_spans_a_split_rows_cells) {
+  plot::Figure figure = split_figure({2, 1});
+  figure.set_share_row_y(true);
+  figure.layout();
+  // The pointer in the *second* cell of the split row, half-way down it.
+  const plot::Rect cell1 = figure.layout().cells[1];
+  const plot::Transform ty = figure.plot(1u).y_transform(0u);
+  const plot::f32 y = static_cast<plot::f32>(ty.to_data_y(cell1.y + 0.5f * cell1.h));
+  figure.set_crosshair(50.0, 1u, y, true);
+  figure.layout();
+  const plot::CrosshairLayout& left = figure.plot(0u).layout().crosshair;
+  const plot::CrosshairLayout& right = figure.plot(1u).layout().crosshair;
+  const plot::CrosshairLayout& other = figure.plot(2u).layout().crosshair;
+  // Both cells of the row carry the line, at one screen row: they share the y scale, so the
+  // row's own y is the same value's height in each.
+  PLOT_CHECK(left.horizontal.w > 0.0f);
+  PLOT_CHECK(right.horizontal.w > 0.0f);
+  PLOT_CHECK_NEAR(left.horizontal.y + 0.5f * left.horizontal.h,
+                  right.horizontal.y + 0.5f * right.horizontal.h, 0.75);
+  // The unsplit row keeps the shared-x vertical line and no horizontal one -- a line there
+  // would mean a value on a scale that row does not have.
+  PLOT_CHECK(other.vertical.w > 0.0f);
+  PLOT_CHECK(other.horizontal.h == 0.0f);
+  // The spread does not touch the x: both cells keep the one shared column.
+  PLOT_CHECK(left.vertical.w > 0.0f);
+  PLOT_CHECK(right.vertical.w > 0.0f);
+  // Without `set_share_row_y` the cells are separate scales again, and the line is the
+  // hovered cell's alone.
+  figure.set_share_row_y(false);
+  figure.set_crosshair(50.0, 1u, y, true);
+  figure.layout();
+  PLOT_CHECK(figure.plot(1u).layout().crosshair.horizontal.w > 0.0f);
+  PLOT_CHECK(figure.plot(0u).layout().crosshair.horizontal.h == 0.0f);
+}
+
+PLOT_TEST(figure_row_y_auto_fits_the_union_and_ignores_empty_cells) {
+  plot::Figure figure = split_figure({3});
+  for (plot::u32 i = 0; i < 3u; ++i) figure.plot(i).set_y_auto_scale(0u, true);
+  plot::data::ArraySeries a(std::vector<plot::f32>{0.0f, 10.0f, 5.0f}, 0.0, 1.0);
+  plot::data::ArraySeries b(std::vector<plot::f32>{-20.0f, 5.0f, 0.0f}, 0.0, 1.0);
+  figure.plot(0u).add_trace({&a, plot::Color::hex(0x40C0FF), 1.5f, 0u, 0u});
+  figure.plot(1u).add_trace({&b, plot::Color::hex(0xFF8080), 1.5f, 0u, 0u});
+  figure.set_share_row_y(true);  // cell 2 has no trace
+  figure.layout();
+  PLOT_CHECK(figure.row_y_auto(0u));
+  const plot::Range r0 = figure.plot(0u).y_range(0u);
+  PLOT_CHECK(r0.min <= -20.0 && r0.max >= 10.0);
+  PLOT_CHECK(r0.max - r0.min < 40.0);  // padded, not blown up
+  for (plot::u32 i = 1; i < 3u; ++i) {
+    PLOT_CHECK_NEAR(figure.plot(i).y_range(0u).min, r0.min, 1e-9);
+    PLOT_CHECK_NEAR(figure.plot(i).y_range(0u).max, r0.max, 1e-9);
+  }
+}
+
+PLOT_TEST(figure_row_y_manual_then_fit_returns_to_auto) {
+  plot::Figure figure = split_figure({2});
+  for (plot::u32 i = 0; i < 2u; ++i) figure.plot(i).set_y_auto_scale(0u, true);
+  plot::data::ArraySeries a(std::vector<plot::f32>{0.0f, 10.0f}, 0.0, 1.0);
+  plot::data::ArraySeries b(std::vector<plot::f32>{-4.0f, 4.0f}, 0.0, 1.0);
+  figure.plot(0u).add_trace({&a, plot::Color::hex(0x40C0FF), 1.5f, 0u, 0u});
+  figure.plot(1u).add_trace({&b, plot::Color::hex(0xFF8080), 1.5f, 0u, 0u});
+  figure.set_share_row_y(true);
+  figure.layout();
+  figure.plot(1u).set_y_range(0u, {1.0, 2.0});
+  figure.layout();
+  PLOT_CHECK(!figure.row_y_auto(0u));
+  PLOT_CHECK_NEAR(figure.plot(0u).y_range(0u).min, 1.0, 1e-9);
+  PLOT_CHECK_NEAR(figure.plot(0u).y_range(0u).max, 2.0, 1e-9);
+  figure.layout();  // manual holds: no fight with the plots' own autoscale
+  PLOT_CHECK_NEAR(figure.plot(1u).y_range(0u).max, 2.0, 1e-9);
+  figure.plot(0u).auto_fit_y(0u);  // what fit-all / double-click does to one cell
+  figure.layout();
+  PLOT_CHECK(figure.row_y_auto(0u));
+  PLOT_CHECK(figure.plot(1u).y_range(0u).min <= -4.0 && figure.plot(1u).y_range(0u).max >= 10.0);
+  PLOT_CHECK_NEAR(figure.plot(0u).y_range(0u).min, figure.plot(1u).y_range(0u).min, 1e-9);
+}
+
+PLOT_TEST(figure_row_y_sharing_off_keeps_cells_independent) {
+  plot::Figure figure = split_figure({2});
+  figure.layout();
+  figure.plot(1u).set_y_range(0u, {-3.0, 7.0});
+  figure.layout();
+  PLOT_CHECK_NEAR(figure.plot(0u).y_range(0u).max, 1.0, 1e-9);
+  PLOT_CHECK(!figure.share_row_y());
+}
+
+PLOT_TEST(figure_row_y_only_the_first_cell_has_a_gutter) {
+  plot::Figure figure = split_figure({1, 3}, false);
+  for (plot::u32 i = 0; i < figure.plot_count(); ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_share_row_y(true);
+  figure.layout();
+  PLOT_CHECK(figure.plot(0u).layout().y_axis_width > 0.0f);   // unsplit row keeps its axis
+  PLOT_CHECK(figure.plot(1u).layout().y_axis_width > 0.0f);   // first cell of the split row
+  for (plot::u32 i = 2; i < 4u; ++i) {
+    const plot::PlotLayout& l = figure.plot(i).layout();
+    PLOT_CHECK(l.y_axis_width == 0.0f && l.y_title_width == 0.0f && l.y_label_area.w == 0.0f);
+    PLOT_CHECK(l.axes.w > 0.0f);
+  }
+  // The gutter is one figure-wide value: the unsplit row and the split row's first cell agree.
+  PLOT_CHECK_NEAR(figure.plot(0u).layout().axes.x - figure.layout().cells[0].x,
+                  figure.plot(1u).layout().axes.x - figure.layout().cells[1].x, 0.01);
+}
+
+PLOT_TEST(figure_row_y_equal_weights_give_equal_plot_widths_and_cells_tile) {
+  plot::Figure figure = split_figure({3}, false);
+  for (plot::u32 i = 0; i < 3u; ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_col_gap(8.0f);
+  figure.set_share_row_y(true);
+  const plot::FigureLayout& fl = figure.layout();
+  const double w0 = figure.plot(0u).layout().axes.w;
+  PLOT_CHECK_NEAR(figure.plot(1u).layout().axes.w, w0, 0.6);
+  PLOT_CHECK_NEAR(figure.plot(2u).layout().axes.w, w0, 0.6);
+  PLOT_CHECK_NEAR(fl.cells[0].x, fl.rows[0].x, 0.001);
+  PLOT_CHECK_NEAR(fl.cells[2].right(), fl.rows[0].right(), 0.001);
+  PLOT_CHECK_NEAR(fl.cells[1].x - fl.cells[0].right(), 8.0, 0.01);
+  PLOT_CHECK_NEAR(fl.cells[2].x - fl.cells[1].right(), 8.0, 0.01);
+}
+
+PLOT_TEST(figure_row_y_weights_apply_to_plot_widths) {
+  plot::Figure figure = split_figure({2}, false);
+  for (plot::u32 i = 0; i < 2u; ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_col_gap(0.0f);
+  figure.set_share_row_y(true);
+  const std::vector<plot::f32> w{3.0f, 1.0f};
+  PLOT_CHECK(figure.set_cell_weights(0u, w));
+  figure.layout();
+  const double a0 = figure.plot(0u).layout().axes.w;
+  const double a1 = figure.plot(1u).layout().axes.w;
+  PLOT_CHECK_NEAR(a0 - 64.0, (a1 - 64.0) * 3.0, 1.0);
+}
+
+PLOT_TEST(figure_row_y_narrow_row_degrades_without_negative_rects) {
+  plot::Figure figure = split_figure({4}, false);
+  figure.set_viewport(plot::Rect{0.0f, 0.0f, 30.0f, 100.0f});
+  for (plot::u32 i = 0; i < 4u; ++i) give_magnitude_row(figure.plot(i), 0u);
+  figure.set_col_gap(10.0f);
+  figure.set_share_row_y(true);
+  double sum = 0.0;
+  for (const plot::Rect& c : figure.layout().cells) {
+    PLOT_CHECK(c.w >= 0.0f && c.h >= 0.0f);
+    sum += c.w;
+  }
+  PLOT_CHECK_NEAR(sum, 30.0, 0.05);
+}
+
+PLOT_TEST(figure_row_y_sharing_off_still_gives_every_cell_a_gutter) {
+  plot::Figure figure = split_figure({2}, false);
+  give_magnitude_row(figure.plot(0u), 0u);
+  give_magnitude_row(figure.plot(1u), 1u);
+  figure.layout();
+  PLOT_CHECK(figure.plot(1u).layout().y_axis_width > 0.0f);
+}
+
+// ------------------------------------------------- x axes follow the columns
+
+namespace {
+
+/// A split figure with a text backend and a y axis of a real magnitude on every plot (so the
+/// shared y gutter is non-zero), the host's configuration: row-shared y, a 6 px column gap and
+/// a 35 px right gutter.
+plot::Figure host_like_figure(const std::vector<plot::u32>& cells, bool share_y = true) {
+  plot::Figure figure = split_figure(cells, false);
+  for (plot::u32 i = 0; i < figure.plot_count(); ++i) give_magnitude_row(figure.plot(i), i);
+  figure.set_col_gap(6.0f);
+  figure.set_right_gutter(35.0f);
+  figure.set_share_row_y(share_y);
+  return figure;
+}
+
+bool shows_x_axis(plot::Figure& figure, plot::u32 plot_index) {
+  const plot::PlotLayout& l = figure.plot(plot_index).layout();
+  return l.x_axis_height > 0.0f && l.x_label_area.h > 0.0f;
+}
+
+bool hides_x_axis(plot::Figure& figure, plot::u32 plot_index) {
+  const plot::PlotLayout& l = figure.plot(plot_index).layout();
+  return l.x_axis_height == 0.0f && l.x_label_area.h == 0.0f;
+}
+
+}  // namespace
+
+PLOT_TEST(figure_x_axis_a_split_row_over_an_unsplit_one_keeps_its_own) {
+  plot::Figure figure = host_like_figure({2, 1});
+  figure.layout();
+  PLOT_CHECK(shows_x_axis(figure, 0u));
+  PLOT_CHECK(shows_x_axis(figure, 1u));
+  PLOT_CHECK(shows_x_axis(figure, 2u));
+  // ... and the other way round.
+  plot::Figure other = host_like_figure({1, 2});
+  other.layout();
+  PLOT_CHECK(shows_x_axis(other, 0u));
+  PLOT_CHECK(shows_x_axis(other, 1u));
+  PLOT_CHECK(shows_x_axis(other, 2u));
+}
+
+PLOT_TEST(figure_x_axis_rows_with_the_same_columns_share_the_bottom_axis) {
+  plot::Figure figure = host_like_figure({2, 2});
+  figure.layout();
+  PLOT_CHECK(hides_x_axis(figure, 0u));
+  PLOT_CHECK(hides_x_axis(figure, 1u));
+  PLOT_CHECK(shows_x_axis(figure, 2u));
+  PLOT_CHECK(shows_x_axis(figure, 3u));
+  // The hidden row's cells have exactly the x extents of the cells below, so the axis below
+  // serves them.
+  const plot::FigureLayout& fl = figure.layout();
+  for (plot::u32 c = 0; c < 2u; ++c) {
+    PLOT_CHECK_NEAR(fl.cells[c].x, fl.cells[2u + c].x, 1e-4);
+    PLOT_CHECK_NEAR(fl.cells[c].w, fl.cells[2u + c].w, 1e-4);
+    const plot::PlotLayout& up = figure.plot(c).layout();
+    const plot::PlotLayout& down = figure.plot(2u + c).layout();
+    PLOT_CHECK_NEAR(up.axes.x, down.axes.x, 1e-4);
+    PLOT_CHECK_NEAR(up.axes.w, down.axes.w, 1e-4);
+    PLOT_CHECK_NEAR(up.y_axis_width, down.y_axis_width, 1e-4);
+  }
+  // The same without row-shared y (every cell has its own y strip).
+  plot::Figure own_y = host_like_figure({2, 2}, false);
+  own_y.layout();
+  PLOT_CHECK(hides_x_axis(own_y, 0u) && hides_x_axis(own_y, 1u));
+  PLOT_CHECK(shows_x_axis(own_y, 2u) && shows_x_axis(own_y, 3u));
+  for (plot::u32 c = 0; c < 2u; ++c) {
+    PLOT_CHECK_NEAR(own_y.plot(c).layout().axes.x, own_y.plot(2u + c).layout().axes.x, 1e-4);
+    PLOT_CHECK_NEAR(own_y.plot(c).layout().axes.w, own_y.plot(2u + c).layout().axes.w, 1e-4);
+  }
+}
+
+PLOT_TEST(figure_x_axis_matching_compares_normalised_weights) {
+  const std::vector<plot::f32> one_three{1.0f, 3.0f};
+  const std::vector<plot::f32> two_six{2.0f, 6.0f};
+  const std::vector<plot::f32> halves{1.0f, 1.0f};
+  {  // 1:3 over 1:3
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    PLOT_CHECK(figure.set_cell_weights(1u, one_three));
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+    PLOT_CHECK(shows_x_axis(figure, 2u) && shows_x_axis(figure, 3u));
+    const plot::FigureLayout& fl = figure.layout();
+    PLOT_CHECK_NEAR(fl.cells[0].w, fl.cells[2].w, 1e-3);
+    PLOT_CHECK_NEAR(figure.plot(1u).layout().axes.x, figure.plot(3u).layout().axes.x, 1e-3);
+  }
+  {  // 1:3 over 2:6 -- normalised, so the same columns
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    PLOT_CHECK(figure.set_cell_weights(1u, two_six));
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+  }
+  {  // 1:3 over 1:1
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    PLOT_CHECK(figure.set_cell_weights(1u, halves));
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u) && shows_x_axis(figure, 1u));
+    PLOT_CHECK(shows_x_axis(figure, 2u) && shows_x_axis(figure, 3u));
+  }
+  {  // 1:3 over (no weights = equal)
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, one_three));
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u) && shows_x_axis(figure, 1u));
+  }
+  {  // explicit 1:1 over (no weights = equal)
+    plot::Figure figure = host_like_figure({2, 2});
+    PLOT_CHECK(figure.set_cell_weights(0u, halves));
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+  }
+  {  // a different number of cells never matches, whatever the weights
+    plot::Figure figure = host_like_figure({3, 2});
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u) && shows_x_axis(figure, 2u));
+  }
+}
+
+PLOT_TEST(figure_x_axis_unsplit_rows_are_unchanged) {
+  plot::Figure figure = host_like_figure({1, 1, 1});
+  figure.layout();
+  PLOT_CHECK(hides_x_axis(figure, 0u));
+  PLOT_CHECK(hides_x_axis(figure, 1u));
+  PLOT_CHECK(shows_x_axis(figure, 2u));
+  PLOT_CHECK_NEAR(figure.layout().x_axis_strip.h, figure.plot(2u).layout().x_label_area.h, 1e-4);
+}
+
+PLOT_TEST(figure_x_axis_follows_a_chain_of_matching_rows) {
+  {  // [2]/[2]/[1]: row 0 matches row 1 and hides; row 1 does not match row 2 and shows.
+    plot::Figure figure = host_like_figure({2, 2, 1});
+    figure.layout();
+    PLOT_CHECK(hides_x_axis(figure, 0u) && hides_x_axis(figure, 1u));
+    PLOT_CHECK(shows_x_axis(figure, 2u) && shows_x_axis(figure, 3u));
+    PLOT_CHECK(shows_x_axis(figure, 4u));
+  }
+  {  // [2]/[2]/[2]: transitive -- only the last row carries the axis.
+    plot::Figure figure = host_like_figure({2, 2, 2});
+    figure.layout();
+    for (plot::u32 i = 0; i < 4u; ++i) PLOT_CHECK(hides_x_axis(figure, i));
+    PLOT_CHECK(shows_x_axis(figure, 4u) && shows_x_axis(figure, 5u));
+  }
+  {  // [1]/[2]/[2]: the unsplit row matches nothing below it.
+    plot::Figure figure = host_like_figure({1, 2, 2});
+    figure.layout();
+    PLOT_CHECK(shows_x_axis(figure, 0u));
+    PLOT_CHECK(hides_x_axis(figure, 1u) && hides_x_axis(figure, 2u));
+    PLOT_CHECK(shows_x_axis(figure, 3u) && shows_x_axis(figure, 4u));
+  }
+  {  // the figure's x_axis_strip is still the last plot's label area
+    plot::Figure figure = host_like_figure({2, 1});
+    const plot::FigureLayout& fl = figure.layout();
+    PLOT_CHECK(fl.x_axis_strip.h > 0.0f);
+    PLOT_CHECK_NEAR(fl.x_axis_strip.h, figure.plot(2u).layout().x_label_area.h, 1e-4);
+  }
+}
+
+PLOT_TEST(figure_x_axis_labels_on_every_row_when_asked) {
+  plot::Figure figure = host_like_figure({2, 2, 1});
+  figure.set_x_labels_only_on_bottom(false);
+  figure.layout();
+  for (plot::u32 i = 0; i < figure.plot_count(); ++i) PLOT_CHECK(shows_x_axis(figure, i));
+}
+
+// -------------------------------------------- the gap between cells of a row
+
+PLOT_TEST(figure_split_row_gap_between_plot_frames_is_the_column_gap) {
+  for (const plot::u32 n : {2u, 3u, 4u}) {
+    plot::Figure figure = host_like_figure({n});
+    const plot::FigureLayout& fl = figure.layout();
+    PLOT_CHECK_NEAR(fl.col_gap, 6.0, 1e-4);
+    for (plot::u32 k = 0; k + 1u < n; ++k) {
+      PLOT_CHECK_NEAR(figure.plot(k + 1u).layout().axes.x - figure.plot(k).layout().axes.right(),
+                      6.0, 1e-3);
+      PLOT_CHECK_NEAR(fl.cells[k + 1u].x - fl.cells[k].right(), 6.0, 1e-3);
+      // Only the last cell holds the right gutter.
+      PLOT_CHECK_NEAR(figure.plot(k).layout().right_gutter, 0.0, 1e-6);
+    }
+    // Equal weights give equal axes widths, the first cell included.
+    for (plot::u32 k = 1; k < n; ++k) {
+      PLOT_CHECK_NEAR(figure.plot(k).layout().axes.w, figure.plot(0u).layout().axes.w, 0.6);
+    }
+    // The cells tile the row exactly.
+    PLOT_CHECK_NEAR(fl.cells[0].x, fl.rows[0].x, 1e-4);
+    PLOT_CHECK_NEAR(fl.cells[n - 1u].right(), fl.rows[0].right(), 1e-3);
+    // The right gutter is still there, on the last cell.
+    const plot::PlotLayout& last = figure.plot(n - 1u).layout();
+    PLOT_CHECK_NEAR(fl.cells[n - 1u].right() - last.axes.right(), 35.0, 1e-3);
+    PLOT_CHECK_NEAR(last.right_gutter, 35.0, 1e-3);
+    // The y gutter still sits on the first cell alone.
+    PLOT_CHECK(figure.plot(0u).layout().axes.x > fl.cells[0].x);
+    PLOT_CHECK_NEAR(figure.plot(1u).layout().axes.x, fl.cells[1].x, 1e-4);
+  }
+}
+
+PLOT_TEST(figure_split_row_gap_with_weights_and_a_y_gutter) {
+  plot::Figure figure = host_like_figure({1, 2});
+  const std::vector<plot::f32> w{3.0f, 1.0f};
+  PLOT_CHECK(figure.set_cell_weights(1u, w));
+  const plot::FigureLayout& fl = figure.layout();
+  const plot::Rect a0 = figure.plot(1u).layout().axes;
+  const plot::Rect a1 = figure.plot(2u).layout().axes;
+  PLOT_CHECK_NEAR(a1.x - a0.right(), 6.0, 1e-3);
+  PLOT_CHECK_NEAR(a0.w - 64.0, (a1.w - 64.0) * 3.0, 1.0);
+  PLOT_CHECK_NEAR(fl.cells[2].right(), fl.rows[1].right(), 1e-3);
+  PLOT_CHECK_NEAR(fl.cells[1].x, fl.rows[1].x, 1e-4);
+  PLOT_CHECK_NEAR(fl.cells[2].right() - a1.right(), 35.0, 1e-3);
+  // The unsplit row above keeps its own right gutter on its one plot.
+  PLOT_CHECK_NEAR(fl.cells[0].right() - figure.plot(0u).layout().axes.right(), 35.0, 1e-3);
+}
+
+PLOT_TEST(figure_split_row_gutters_degrade_on_a_narrow_row) {
+  plot::Figure figure = host_like_figure({3});
+  figure.set_viewport(plot::Rect{0.0f, 0.0f, 60.0f, 100.0f});
+  const plot::FigureLayout& fl = figure.layout();
+  double sum = 0.0;
+  for (const plot::Rect& c : fl.cells) {
+    PLOT_CHECK(c.w >= 0.0f && c.h >= 0.0f);
+    sum += c.w;
+  }
+  PLOT_CHECK_NEAR(sum + fl.col_gap * 2.0, 60.0, 0.05);
+  PLOT_CHECK_NEAR(fl.cells[2].right(), 60.0, 0.05);
+  for (plot::u32 i = 0; i < 3u; ++i) PLOT_CHECK(figure.plot(i).layout().axes.w >= 0.0f);
+}
+
+PLOT_TEST(figure_row_gutter_without_sharing_is_the_old_layout) {
+  plot::Figure figure = host_like_figure({2}, false);
+  const plot::FigureLayout& fl = figure.layout();
+  // Plain cells: equal halves of the row less the gap, every plot with its own right gutter.
+  PLOT_CHECK_NEAR(fl.cells[0].w, (fl.rows[0].w - 6.0) / 2.0, 1e-3);
+  PLOT_CHECK_NEAR(fl.cells[1].w, (fl.rows[0].w - 6.0) / 2.0, 1e-3);
+  PLOT_CHECK_NEAR(fl.cells[1].x - fl.cells[0].right(), 6.0, 1e-4);
+  for (plot::u32 i = 0; i < 2u; ++i) {
+    PLOT_CHECK_NEAR(figure.plot(i).layout().right_gutter, 35.0, 1e-3);
+    PLOT_CHECK_NEAR(fl.cells[i].right() - figure.plot(i).layout().axes.right(), 35.0, 1e-3);
+  }
+  // An unsplit row is unchanged by sharing being on.
+  plot::Figure unsplit = host_like_figure({1, 1});
+  const plot::FigureLayout& ul = unsplit.layout();
+  for (plot::u32 i = 0; i < 2u; ++i) {
+    PLOT_CHECK_NEAR(ul.cells[i].w, ul.rows[i].w, 1e-4);
+    PLOT_CHECK_NEAR(ul.cells[i].right() - unsplit.plot(i).layout().axes.right(), 35.0, 1e-3);
+  }
+}

tests/test_render_smoke.cpp

@@ -5059,6 +5059,52 @@ PLOT_TEST(figure_x_title_is_drawn_on_the_bottom_row_only) {
 
   ctx.destroy();
 }
+/// A split row that shares one y axis draws it once, on its first cell: every later cell is
+/// `YStrip::None`, reserves no y strip, and the renderer must stage no y tick labels for it.
+/// Staged text is not observable through `FrameStats`, so this reads the pixels. The column gap
+/// is wide and empty chrome, and a label staged for the second cell would be right-aligned
+/// against that cell's (zero-width) label area, i.e. land in the gap: the gap holds no ink.
+PLOT_TEST(figure_shared_y_second_cell_stages_no_y_labels) {
+  plottest::HeadlessContext ctx(kW, kH);
+  plot::Status st;
+  PLOT_CHECK(ctx.create(&st));
+  PLOT_CHECK(plot::gl::load(nullptr));
+  plot::Renderer r;
+  PLOT_CHECK(r.init(&st));
+
+  const std::vector<plot::u32> cells{2u};
+  plot::Figure figure{std::span<const plot::u32>(cells)};
+  figure.set_viewport({0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  for (plot::u32 i = 0; i < 2u; ++i) {
+    plot::Plot& p = figure.plot(i);
+    p.add_x_axis({"t", {0.0, 100.0}, false, plot::AxisPlace::Bottom, false});
+    p.add_y_axis({"", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
+  }
+  figure.set_col_gap(80.0f);
+  figure.set_share_row_y(true);
+
+  r.begin_frame(kW, kH, {0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  r.draw(figure);
+  r.end_frame();
+  PLOT_CHECK(!r.last_frame_had_gl_error());
+  const plot::FigureLayout layout = figure.layout();
+  const plot::Color ink = r.style().axis_text;
+
+  const plot::PlotLayout& first = figure.plot(0u).layout();
+  const plot::PlotLayout& second = figure.plot(1u).layout();
+  PLOT_CHECK(second.y_label_area.w == 0.0f);
+  PLOT_CHECK(layout.cells[1].x - layout.cells[0].right() > 70.0f);
+  // The first cell still draws the row's one axis ...
+  PLOT_CHECK(scan_rect_ink(ctx, first.y_label_area, ink).pixels > 0u);
+  // ... and nothing is staged in the gap on the second cell's left.
+  // Inset a few pixels each side: the cells' own frame lines sit on the gap's two edges.
+  const plot::Rect gap{layout.cells[0].right() + 4.0f, first.axes.y,
+                       layout.cells[1].x - layout.cells[0].right() - 8.0f, first.axes.h};
+  PLOT_CHECK(scan_rect_ink(ctx, gap, ink).pixels == 0u);
+
+  ctx.destroy();
+}
+
 // ---------------------------------------------------- the axes clip (spec §13)
 
 // The reported defect this section is about: a trace is staged in *data* space and
@@ -5704,17 +5750,22 @@ PLOT_TEST(horizontal_ruler_label_reads_its_value) {
 
   const InkExtent ink = ink_extent(frame, kW, kH, ruler_pixel);
   PLOT_CHECK(ink.pixels > 0u);
-  // One horizontal run: shallow in rows, spanning essentially the axes' own width.
+  // One horizontal run: the stroke itself is shallow, but the label's backplate is drawn in
-  PLOT_CHECK(ink.rows() <= 3u);
+  // the ruler's own colour too (see the clustered cursor labels), and its border spans one
+  // text line plus its padding above and below the stroke -- measured 28 rows on the suite's
+  // own font. The bound is that panel with a stroke's worth of slack, not the old bare-text
+  // run of three.
+  PLOT_CHECK(ink.rows() <= 32u);
   PLOT_CHECK(ink.cols() + 2u >= static_cast<plot::u32>(axes.w));
   // And at the row the transform says -- a ruler drawn at the wrong y, or a vertical column
   // that happens to be inside the rect, cannot satisfy both of these.
   PLOT_CHECK(static_cast<plot::f32>(ink.top) <= expected_row + 1.0f);
   PLOT_CHECK(static_cast<plot::f32>(ink.bottom) >= expected_row - 1.0f);
 
-  // The label: text ink just under the ruler at its left end, inside the axes. The tick
+  // The label: text ink centred on the ruler at its left end, inside the axes. The tick
   // labels are outside `axes`, in their own strips, so this can only be the ruler's value.
-  const plot::Rect label{axes.x + 1.0f, expected_row, 60.0f, 16.0f};
+  // Centred, not below: the badge straddles the ruler's own row (see the clustered labels).
+  const plot::Rect label{axes.x + 1.0f, expected_row - 9.0f, 60.0f, 18.0f};
   PLOT_CHECK(predicate_pixels(frame, kW, label, text_pixel) > 0u);
 
   // The clip: every ruler-coloured pixel of the frame is inside `axes`.
@@ -6050,15 +6101,65 @@ PLOT_TEST(crosshair_ink_stays_inside_the_axes) {
   PLOT_CHECK(crosshair.rows[2].marker.x < axes.x);
   PLOT_CHECK(crosshair.rows[2].marker.x >= layout.y_label_area.x);
 
-  // Every crosshair-coloured pixel of the frame is inside `axes`, and there are some: not one in a
+  // Every crosshair-coloured pixel of the frame is inside `axes` or inside one of the two
-  // label or title strip, not one over the host's surface outside the viewport.
+  // axis badges, and there are some. The badges are the deliberate exception -- the
+  // crosshair's own axis readings, drawn in the strips the ticks are written in (see
+  // `Crosshair::axis_labels`) -- and the layout named their rects, so the exception is
+  // checked against exactly what was placed, not against a loose region.
+  PLOT_CHECK(crosshair.y_axis_badge.w > 0.0f);
+  PLOT_CHECK(crosshair.x_axis_badge.w > 0.0f);
+  PLOT_CHECK(crosshair.y_axis_badge.right() <= axes.x + 0.5f);  // beside the y ticks
+  PLOT_CHECK(crosshair.x_axis_badge.y >= axes.bottom() - 0.5f); // beside the x ticks
   const plot::u32 inside = predicate_pixels(frame, kW, axes, crosshair_pixel);
+  // The badge rects are the panels' own frames, and a hairline's anti-aliased fringe can sit
+  // half a pixel outside one -- the badge's own ink either way, so the counts read over the
+  // rects inflated by a pixel. The inflation is held out of the axes: a badge sits *against*
+  // an axis line, and a fringe column read into the axes' own pixels would be counted twice.
+  const auto inflate_badge = [&axes](const plot::Rect& rect, bool toward_axes) {
+    plot::Rect out{rect.x - 1.0f, rect.y - 1.0f, rect.w + 2.0f, rect.h + 2.0f};
+    if (toward_axes && out.right() > axes.x) out.w -= out.right() - axes.x;
+    if (!toward_axes && out.y < axes.bottom()) {
+      const f32 d = axes.bottom() - out.y;
+      out.y += d;
+      out.h -= d;
+    }
+    return out;
+  };
+  const plot::Rect y_badge_rect = inflate_badge(crosshair.y_axis_badge, true);
+  const plot::Rect x_badge_rect = inflate_badge(crosshair.x_axis_badge, false);
+  const plot::u32 y_badge = predicate_pixels(frame, kW, y_badge_rect, crosshair_pixel);
+  const plot::u32 x_badge = predicate_pixels(frame, kW, x_badge_rect, crosshair_pixel);
   PLOT_CHECK(inside > 0u);
-  PLOT_CHECK(predicate_pixels(frame, kW, kFrameRect, crosshair_pixel) == inside);
+  PLOT_CHECK(y_badge > 0u);
-  PLOT_CHECK(predicate_pixels(frame, kW, layout.y_label_area, crosshair_pixel) == 0u);
+  PLOT_CHECK(x_badge > 0u);
-  PLOT_CHECK(predicate_pixels(frame, kW, layout.y_title_area, crosshair_pixel) == 0u);
+  PLOT_CHECK(predicate_pixels(frame, kW, kFrameRect, crosshair_pixel) ==
-  PLOT_CHECK(predicate_pixels(frame, kW, layout.x_label_area, crosshair_pixel) == 0u);
+             inside + y_badge + x_badge);
-  PLOT_CHECK(predicate_pixels(frame, kW, layout.x_title_area, crosshair_pixel) == 0u);
+  // The badges may reach past the label strip into the title strip beside it -- a badge is as
+  // wide as its own text and is capped by nothing but the strips' union -- so the title
+  // strips' rule is: **every crosshair-coloured pixel is the badge's own**, i.e. within one
+  // badge rect's one-pixel fringe. A stray pixel there is a second drawing in a strip that
+  // belongs to the axis title.
+  const auto title_stray = [&](const plot::Rect& strip, const plot::Rect& badge) {
+    const plot::Rect frame_rect{badge.x - 1.0f, badge.y - 1.0f, badge.w + 2.0f, badge.h + 2.0f};
+    plot::u32 stray = 0;
+    for (plot::i32 y = static_cast<plot::i32>(strip.y);
+         y < static_cast<plot::i32>(strip.bottom()); ++y) {
+      for (plot::i32 x = static_cast<plot::i32>(strip.x);
+           x < static_cast<plot::i32>(strip.right()); ++x) {
+        const plot::f32 fx = static_cast<plot::f32>(x);
+        const plot::f32 fy = static_cast<plot::f32>(y);
+        const plot::u8* pixel =
+            &frame[(static_cast<plot::usize>(y) * kW + static_cast<plot::usize>(x)) * 4u];
+        if (!crosshair_pixel(pixel)) continue;
+        const bool own = fx >= frame_rect.x && fx < frame_rect.x + frame_rect.w &&
+                         fy >= frame_rect.y && fy < frame_rect.y + frame_rect.h;
+        if (!own) ++stray;
+      }
+    }
+    return stray;
+  };
+  PLOT_CHECK(title_stray(layout.y_title_area, crosshair.y_axis_badge) == 0u);
+  PLOT_CHECK(title_stray(layout.x_title_area, crosshair.x_axis_badge) == 0u);
   plot::u8 host[4] = {};
   PLOT_CHECK(ctx.read_pixel(4u, kH - 4u, host));
   PLOT_CHECK(host[0] > 100 && host[0] < 160 && host[1] > 100 && host[2] > 100);
@@ -6087,6 +6188,21 @@ PLOT_TEST(crosshair_ink_stays_inside_the_axes) {
   PLOT_CHECK(predicate_pixels(frame, kW, kFrameRect, magenta_pixel) ==
              predicate_pixels(frame, kW, axes, magenta_pixel));
 
+  // The same plot with the badges off: the layout places none, and the strips go back to
+  // being clean of the crosshair's colour -- the flag is the whole difference between the
+  // two frames, and this leg is what makes it a setting rather than a behaviour.
+  p.set_crosshair_axis_labels(false);
+  p.layout();
+  r.begin_frame(kW, kH, vp);
+  r.draw(p);
+  r.end_frame();
+  PLOT_CHECK(!r.last_frame_had_gl_error());
+  const std::vector<plot::u8> off_frame = read_frame(ctx);
+  PLOT_CHECK(p.layout().crosshair.y_axis_badge.w == 0.0f);
+  PLOT_CHECK(p.layout().crosshair.x_axis_badge.w == 0.0f);
+  PLOT_CHECK(predicate_pixels(off_frame, kW, layout.y_label_area, crosshair_pixel) == 0u);
+  PLOT_CHECK(predicate_pixels(off_frame, kW, layout.x_label_area, crosshair_pixel) == 0u);
+
   ctx.destroy();
 }
 
@@ -6232,6 +6348,13 @@ PLOT_TEST(crosshair_dash_is_configurable_and_zero_off_means_solid) {
   // 4 on / 4 off over the axes' width: many dashes, not one line.
   const plot::u32 dashed = runs(4.0f);
   PLOT_CHECK(dashed > 4u);
+
+  // The y axis badge is **centred** on the line it names, the way the x badge is centred on
+  // its column -- its middle is the line's own screen row. A badge hung from the line (top
+  // edge on it) is half a badge off, which is exactly what this reads for.
+  const plot::Rect badge = p.layout().crosshair.y_axis_badge;
+  PLOT_CHECK(badge.w > 0.0f);
+  PLOT_CHECK_NEAR(badge.y + 0.5f * badge.h, cyp, 0.6f);
   // Off == 0 is solid: one run, the whole width, no gap anywhere.
   PLOT_CHECK(runs(0.0f) == 1u);
   // A longer gap is a longer period, so fewer dashes over the same width.