fix(plot): a figure row's strip shows labels and title independently, and the shared axis is the bottom row's strip

include/plot/plot/figure.hpp

@@ -24,12 +24,18 @@ struct FigureLayout {
   /// strips and axes come from its own `Plot::layout()` against this rect, so a row
   /// with a y title keeps its column and no two rows can disagree about chrome.
   std::vector<Rect> rows;
-  /// The strip the figure's one shared x axis is drawn in: the full width of the
-  /// figure, directly under the last row, and zero-sized when the figure draws no
-  /// shared axis (`set_x_labels_only_on_bottom(false)`, or no rows). It is outside
-  /// every row rect, because the strip is the figure's -- the rows gave it up
-  /// (`Plot::set_x_label_strip()`), which is what keeps the labels from appearing
-  /// once per row.
+  /// The strip the figure's one shared x axis is drawn in: the bottom row's own
+  /// `PlotLayout::x_label_area`, at the 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
+  /// which case it is simply the bottom row's own strip, one of several.
+  ///
+  /// 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
+  /// the shared axis lives in a row rather than in a band of its own: a strip outside
+  /// every row would be a zero-height rect no press can ever land in.
   Rect x_axis_strip;
   /// The vertical gap between two rows, in device pixels. 0 when the rows tile the
   /// figure edge to edge, which is the default; it is dropped for a figure too
@@ -50,9 +56,12 @@ 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 rows reserve no x label strip of their own and the
-/// figure draws one shared axis under the last row, from the last row's own ticks
-/// and transform: one set of labels, one baseline, no duplication.
+/// on -- the default -- the figure's one x axis is the **bottom row's own**: the rows
+/// above reserve no x chrome at all, and the bottom row keeps its own label and title
+/// strips, so those labels and that title are drawn once, from the bottom row's own
+/// ticks and transform. That places the 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.

include/plot/plot/plot.hpp

@@ -17,6 +17,34 @@
 
 namespace plot {
 
+/// Which parts of a plot's bottom x chrome `layout()` reserves.
+///
+/// The bottom chrome is two strips -- the *label* strip the tick labels live in and
+/// the *title* strip the axis titles live in -- and they are one enum rather than two
+/// flags because a strip means little on its own: a title with no labels needs a row
+/// of its own below where the labels would have been, and a figure's interior rows want
+/// neither. Naming the combinations keeps a caller from switching one off and silently
+/// taking the other with it, which is what a boolean would do.
+///
+/// The one caller that selects a mode is `Figure`: an interior row of a figure that
+/// draws its x labels on the bottom only reserves no x chrome at all (`None`), so its
+/// `axes` reaches its rect's bottom edge and the figure's one x axis -- the bottom row's
+/// own, whose strip the shared axis occupies -- is the figure's only one. A plot drawn
+/// on its own is `Both`, which is what it has always been; `LabelsOnly` and `TitleOnly`
+/// are for a host that wants one without the other.
+enum class XStrip { Both, LabelsOnly, TitleOnly, 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.
+constexpr bool x_strip_has_labels(XStrip mode) noexcept {
+  return mode == XStrip::Both || mode == XStrip::LabelsOnly;
+}
+/// Whether a strip mode reserves the title strip, the same one rule for the title.
+constexpr bool x_strip_has_title(XStrip mode) noexcept {
+  return mode == XStrip::Both || mode == XStrip::TitleOnly;
+}
+
 /// Pixel geometry of a plot's chrome, all of it in the same pixel space as
 /// `Rect` (origin top-left) and therefore in **device** pixels: everything here is
 /// what the renderer draws with, already converted from the logical metrics in
@@ -43,9 +71,8 @@ struct PlotLayout {
   /// Left strip width. The default is used when no text backend has been set.
   f32 y_axis_width = 56.0f;
   /// Bottom strip height. The default is used when no text backend has been set, and
-  /// it is 0.0f when the strip is not reserved at all -- see
-  /// `Plot::set_x_label_strip()`, which is how a row of a `Figure` hands the band to
-  /// the figure's one shared axis.
+  /// it is 0.0f when the label strip is not reserved at all -- see `XStrip`, which is
+  /// how a row of a `Figure` hands the band to the figure's one shared x axis.
   f32 x_axis_height = 22.0f;
   /// Width of `y_title_area` and height of `x_title_area`, both 0.0f when nothing
   /// is reserved. Every titled axis of a direction gets its own text line inside
@@ -134,23 +161,18 @@ class Plot {
   /// The scale the last layout was computed at. 1.0 until a host says otherwise.
   f32 ui_scale() const noexcept { return style_.ui_scale; }
 
-  /// Whether `layout()` reserves the bottom x label strip -- and, with it, the x
-  /// title strip: a title is the labels' caption and goes where they do.
+  /// Which parts of the bottom x chrome `layout()` reserves; `XStrip::Both` by
+  /// default, which is what a plot drawn on its own has always been.
   ///
-  /// True by default, which is what a plot drawn on its own has always been. A
-  /// `Figure` turns it off on every row while it draws one shared x axis under its
-  /// last row (see `Figure::set_x_labels_only_on_bottom()`): the strip is not
-  /// merely left blank, it is not reserved at all, so such a row's `axes` reaches
-  /// the row rect's bottom edge and the row gives the band to the figure's shared
-  /// axis instead of growing a second, duplicate one. Nothing else about the row
-  /// changes -- its ticks still refresh and its grid still builds, because the
-  /// tick list is what the shared axis and the grid are drawn from, not the strip.
-  ///
-  /// The plot it belongs to draws no x tick labels while it is off (their strip
-  /// has no height left to hold them); whoever turned it off is the one that puts
-  /// them somewhere -- `Renderer::draw(Figure&)` draws them in the figure's strip.
-  void set_x_label_strip(bool on) noexcept { x_label_strip_ = on; }
-  bool x_label_strip() const noexcept { return x_label_strip_; }
+  /// A part that is not reserved costs nothing: the strip has no height (or the title
+  /// no row), `axes` grows into the band, and the renderer stages no run for it. A
+  /// figure's interior rows are the case that asked for it -- they sit under one shared
+  /// x axis, so they reserve no x chrome at all -- and `Figure::set_x_labels_only_on_bottom()`
+  /// is what selects their mode. Nothing else about such a row changes: its ticks still
+  /// refresh and its grid still builds, because the tick list is what a grid and a
+  /// shared axis are drawn from, not the strip.
+  void set_x_strip_mode(XStrip mode) noexcept { x_strip_ = mode; }
+  XStrip x_strip_mode() const noexcept { return x_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
@@ -555,8 +577,8 @@ class Plot {
   render::Style style_{};
   const render::TextBackend* text_ = nullptr;
   Legend legend_{};
-  /// Whether the bottom x label strip is reserved at all; see set_x_label_strip().
-  bool x_label_strip_ = true;
+  /// Which parts of the bottom x chrome are reserved at all; see set_x_strip_mode().
+  XStrip x_strip_ = XStrip::Both;
   /// Which handles a pointer is on, published by the input layer (see HotHandles).
   HotHandles hot_{};