feat(plot): a figure can keep an empty band to the right of its data

`layout_.axes` reached the viewport's right edge, so the rightmost x tick
label -- centred on its tick -- had half of itself outside the plot, and
the newest sample's ink landed on the frame. `Plot::set_right_gutter()`
reserves a band there, out of the same `1 - kMinAxesShare` allowance the
label and title strips are served from, so it can no more squeeze `axes`
below the half-viewport floor than they can. `Figure` sets one value on
every row, the left gutter's reason: the rows' axes have to end on a
common column as well as start on one.

Nothing is drawn in the band, so unlike the strips it is a want and never
a measurement: no measuring pass, and it cannot feed the layout's fixed
point.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -47,7 +47,9 @@ run touched a GPU, so read it as an upper bound and a record of the software pat
 - cursors in both orientations — vertical markers and horizontal rulers;
 - draggable regions and measurement bands with exact interval statistics;
 - a signal legend anchored to an axes corner;
-- a hover crosshair with a per-signal readout of the nearest sample's value;
+- a hover crosshair with a per-signal readout, quoting the nearest sample and marking it
+  (`CrosshairValue::NearestSample`, the default) or interpolating between two
+  (`CrosshairValue::Interpolated`);
 - a multi-plot `Figure` of stacked rows sharing one synchronised x axis;
 - live follow (`XFollow::Live`) with relative (multiplicative) zoom and mouse panning;
 - XY series for irregularly sampled data (`plot/data/xy_series.hpp`): sorted-ascending x

docs/data-and-performance.md

@@ -179,18 +179,23 @@ these two is *how many* samples answer the question:
   and no query allocates. `Measure::evaluate` and `Plot::region_stats` are built on these,
   never on the `*_buckets` forms, which would silently report wrong extremes for an
   unaligned interval.
-- **The crosshair readout is a single sample — the nearest one.** `crosshair_span(series,
+- **The crosshair readout is a single sample — the nearest one — by default.**
-  x)` returns the two samples `x` lies between (`begin`, `end`, `t`), and the readout takes
+  `crosshair_span(series, x)` returns the two samples `x` lies between (`begin`, `end`, `t`),
-  whichever of the two is nearer: `t <= 0.5` the earlier, otherwise the later. It answers
+  and `CrosshairValue::NearestSample` takes whichever of the two is nearer: `t <= 0.5` the
-  "which datum is under the pointer", not "what is the signal *at* this instant" — a
+  earlier, otherwise the later. It answers "which datum is under the pointer", not "what is
-  linearly interpolated y is a number the series does not contain, and on a source whose
+  the signal *at* this instant" — a linearly interpolated y is a number the series does not
-  samples are already averaged or decimated at ingest it is a claim about a signal nobody
+  contain, and on a source whose samples are already averaged or decimated at ingest it is a
-  measured. The lookup is O(1) and allocation-free, and it reads level 0, so the value is a
+  claim about a signal nobody measured. That is why it is the default; a host whose x is a
-  stored sample and never an LOD bucket's aggregate; `Plot::crosshair_value(trace, x,
+  quantity it is willing to interpolate states `CrosshairValue::Interpolated` and gets
-  out_y)` exposes the same number to a host, and `CrosshairRow::marker` is placed from that
+  `crosshair_interpolate(y_begin, y_end, t)` instead, at the crosshair's own x. The lookup is
-  same y, so the dot and the number cannot disagree. A row whose trace has no sample at the
+  O(1) and allocation-free either way, and it reads level 0, so the value is a stored sample
-  crosshair's x shows `kCrosshairNoValue` (an em dash, `"—"`), and an x outside the series
+  (or a blend of two) and never an LOD bucket's aggregate. `Plot::crosshair_point(trace, x,
-  window yields no value rather than clamping to the first or last sample.
+  out_x, out_y)` exposes **both** coordinates to a host and `crosshair_value()` the y alone;
+  `CrosshairRow::x`/`y` and its `marker` are placed from that same pair, so in
+  `NearestSample` the mark snaps onto the quoted sample and the dot and the number cannot
+  disagree. A row whose trace has no sample at the crosshair's x shows `kCrosshairNoValue`
+  (an em dash, `"—"`), and an x outside the series window yields no value rather than
+  clamping to the first or last sample.
 
 `Measurement` (from `Plot::measurement()`) documents `integral` as a **left-Riemann sum**,
 `sum(begin, end) * dx` — the samples taken as the value at the left edge of each step. That

docs/interaction.md

@@ -106,6 +106,7 @@ one per plot.
 - `set_crosshair_readout(bool)` switches the panel off — or back on — without moving the
   crosshair. `set_crosshair_style(const CrosshairStyle&)` overrides this plot's stroke and
   dash.
+- `set_crosshair_value_mode(CrosshairValue)` chooses what a readout *is* — see below.
 
 In a `Figure`, the crosshair is applied to **every row** (one shared `x`): each row's readout
 lists its own traces at that instant. `Figure::set_crosshair(x, active)` is vertical-only
@@ -113,10 +114,39 @@ across the figure, and `Figure::set_crosshair(x, hovered_row, y_in_row, active)`
 horizontal line **only in the row the pointer is in** — a figure's rows share x and not y, so
 a figure-wide horizontal line would be a value that means something different in each row.
 
-Readout values are the **nearest sample's**, neither interpolated between two samples nor an
+### What a readout is: the two value modes
-interval statistic (see [data-and-performance.md](data-and-performance.md)); a row whose trace
+
-has no sample at the crosshair's x is listed with the em dash `"—"` and no marker, rather than a
+`Crosshair::value_mode` (`set_crosshair_value_mode()`) answers two different questions, and
-fabricated value.
+which one a caller is asking decides the mode:
+
+| mode | the value | the marker | answers |
+|---|---|---|---|
+| `CrosshairValue::NearestSample` (default) | the nearer bracketing sample's y (a tie reads the earlier) | **snapped to that sample's own x** | *which datum is under the pointer?* |
+| `CrosshairValue::Interpolated` | a linear blend of the two bracketing samples | at the crosshair's own x | *what is the signal at this instant?* |
+
+The default is the sample, because a readout that quotes a number the data does not contain is
+a second estimate laid over the first — and once the view is zoomed past the sample spacing the
+polyline between two samples is a line the *renderer* drew, so reading a value off it reports
+the drawing rather than the signal. A host whose x is a continuous quantity it is willing to
+interpolate (a smooth analytic curve, a resampled series) states `Interpolated` and gets the
+blend, exactly at the pointer. Either way the value is a **stored** sample or a blend of two of
+them, never an LOD bucket's aggregate (see
+[data-and-performance.md](data-and-performance.md)).
+
+`CrosshairRow` carries both halves: `x` and `y` are the data coordinate the value belongs to —
+the quoted sample's own in `NearestSample`, the crosshair's x in `Interpolated` — and `marker`
+is that point in screen space. `Plot::crosshair_point(trace, x, out_x, out_y)` is the same
+lookup for a host that draws instruments of its own; `Plot::crosshair_value()` is it with the x
+discarded.
+
+The mark is drawn `Style::crosshair_marker_px` across (9 logical px) at
+`crosshair_marker_width`, both deliberately **larger than a trace's own sample dots**
+(`marker_px`, 3): in `NearestSample` the mark lands *among* those dots, and at the same size it
+would read as one more sample instead of as the one the panel is quoting. `0` across draws no
+mark.
+
+A row whose trace has no sample at the crosshair's x is listed with the em dash `"—"` and no
+marker, rather than a fabricated value.
 
 ## The gesture table (exactly as implemented)
 

include/plot/data/array_series.hpp

@@ -33,7 +33,7 @@ class ArraySeries final : public Series {
   Range x_bounds() const override;
   Range y_bounds() const override;
   const MinMaxPyramid& pyramid() const override;
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
               GatherMode mode = GatherMode::Envelope) const override;
 
  protected:

include/plot/data/series.hpp

@@ -59,12 +59,20 @@ class Series {
 
   /// Appends interleaved (x, y) pairs for level `level`.
   ///
-  /// **x is relative to `x0()`**: the emitted value is `x_at(i) - x0()` as
+  /// **x is relative to `x_origin`**: the emitted value is `x_at(i) - x_origin` as f32, never
-  /// f32, never the absolute coordinate. A consumer reconstructs the absolute
+  /// the absolute coordinate. A consumer reconstructs the absolute x in f64
-  /// x in f64 (`x0() + emitted`) before mapping it to the screen; staging an
+  /// (`x_origin + emitted`) before mapping it to the screen; staging an absolute epoch-time x
-  /// absolute epoch-time x as f32 would quantise it to a grid coarser than
+  /// as f32 would quantise it to a grid coarser than the pixel (f32 carries 24 bits -- 1 s at
-  /// the pixel (f32 carries 24 bits -- 1 s at 1.7e9), while the relative
+  /// 1.7e9). y is unchanged.
-  /// value `i*dx` is small and exact. y is unchanged.
+  ///
+  /// **The caller names the origin because only the caller knows the span the f32 has to be
+  /// exact against.** The origin was `x0()` -- the start of the series' *retained window* --
+  /// and a relative value's ULP is then a fraction of the whole window however little of it is
+  /// on screen: 1.9 us over 20 s of retained data, which at a view 10 us wide is more than a
+  /// hundred pixels between where a sample is *drawn* and the x a readout quotes for the same
+  /// sample (`x_at()`, exact f64). Passing the visible range's minimum instead makes the error
+  /// 2^-24 of the *visible span* -- a fixed 4e-5 of a pixel over a 700 px axis -- at every
+  /// zoom. A caller with no view in mind may pass `x0()` and get the old behaviour.
   ///
   /// Level 0 emits the raw samples of [begin, end). Level >= 1 emits one
   /// geometry element per *complete* level bucket wholly inside the retained
@@ -85,7 +93,7 @@ class Series {
   /// level_for() picks), and never reaches outside the retained window -- nor
   /// outside [begin, end). Pairs are appended to out_xy, which is never
   /// cleared.
-  virtual void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  virtual void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                       GatherMode mode = GatherMode::Envelope) const = 0;
 
   /// Absolute x of sample `i` (`x0() + i*dx()` on a uniform-grid series).
@@ -121,7 +129,7 @@ class Series {
 
   /// The shared body of the concrete gather() overrides: raw samples at level 0,
   /// bucket geometry above it (per `mode`).
-  void gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  void gather_impl(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                    GatherMode mode) const;
 
  private:
@@ -143,7 +151,7 @@ class Series {
   /// 0 is entirely this, and a level >= 1 gather uses it for the two partial edge
   /// buckets a pyramid of complete buckets cannot answer for. Precondition:
   /// begin < end, end <= size().
-  void gather_raw(u64 begin, u64 end, std::vector<f32>& out_xy) const;
+  void gather_raw(u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy) const;
 };
 
 }  // namespace plot::data

include/plot/data/stream_series.hpp

@@ -92,7 +92,7 @@ class StreamSeries final : public Series {
   Range x_bounds() const override;
   Range y_bounds() const override;
   const MinMaxPyramid& pyramid() const override;
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
               GatherMode mode = GatherMode::Envelope) const override;
   /// Always true: this series' `x_bounds()` is the retained ring window, which
   /// slides with every `acquire()` -- the property `XFollow::Live` follows.

include/plot/data/xy_series.hpp

@@ -53,8 +53,8 @@ class XYSeries final : public Series {
   Range x_bounds() const override;
   Range y_bounds() const override;
   const MinMaxPyramid& pyramid() const override;
-  /// Same contract as `Series::gather`, with x emitted relative to `x0()`.
+  /// Same contract as `Series::gather`, with x emitted relative to `x_origin`.
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
               GatherMode mode = GatherMode::Envelope) const override;
 
   /// The sample's own x.

include/plot/plot/crosshair.hpp

@@ -30,6 +30,35 @@ struct CrosshairStyle {
                          ///< (once resolved) means solid.
 };
 
+/// What a crosshair readout's number *is*: one of the series' own samples, or a value
+/// computed between two of them.
+///
+/// The two answer different questions and the right one is a property of the data, which the
+/// plot cannot know — so it is the caller's to state.
+///
+/// * `NearestSample` answers **"which datum is under the pointer"**. The row reports the y of
+///   whichever bracketing sample is nearer the crosshair's x (a tie reading the earlier), and
+///   the marker is placed at **that sample's own x**, so the mark says *which* sample the
+///   number came from. This is the default, and it is the honest mode for a source whose
+///   samples are already averaged or decimated before they arrive: a number between two of
+///   those is a second estimate of a signal that nothing measured, and at a zoom past the
+///   sample spacing the polyline between two samples is a line the renderer drew, so a
+///   reading off it reports the drawing.
+/// * `Interpolated` answers **"what is the signal at this instant"**, by linear interpolation
+///   between the two bracketing samples (`crosshair_interpolate()`). The marker stays on the
+///   crosshair's own x, on the polyline, because that is where the number is. Right for a
+///   continuous quantity sampled finely enough that the line between two samples is a model
+///   the caller believes — a smooth analytical curve, an interpolating instrument — and
+///   wrong wherever that line is an artefact of plotting.
+///
+/// Set per plot through `Plot::set_crosshair_value_mode()`. It reaches the readout's numbers,
+/// the markers' positions and `Plot::crosshair_value()` alike, so a host and the panel beside
+/// the pointer can never disagree about what the value means.
+enum class CrosshairValue : u8 {
+  NearestSample,  ///< The nearer bracketing sample's y, marked at that sample's x. Default.
+  Interpolated,   ///< A linear blend of the two, marked at the crosshair's own x.
+};
+
 /// The crosshair's state: where it is, which of its two lines it draws, and whether the
 /// per-signal readout is wanted.
 ///
@@ -44,11 +73,10 @@ struct CrosshairStyle {
 /// drawn -- the single-coordinate `Plot::set_crosshair()` leaves the vertical one only,
 /// and a `Figure` is always vertical-only because its rows share x and not y.
 ///
-/// **The readout's values are samples.** The y a row reports for a trace is the y of
+/// **The readout's values are samples by default** -- the y a row reports is the nearer of
-/// whichever of the two samples adjacent to `x` is **nearer** to it -- not a value
+/// the two samples adjacent to `x`, and its marker sits on that sample. `value_mode` is what
-/// interpolated between them, and not an interval statistic. See `crosshair_span()` for the
+/// chooses otherwise; see `CrosshairValue` for the two questions it picks between, and
-/// rule stated in full, and for how it differs from
+/// `crosshair_span()` for how both differ from `Series::reduce()`/`sum()`/`sumsq()`.
-/// `Series::reduce()`/`sum()`/`sumsq()`.
 struct Crosshair {
   /// Whether the crosshair is drawn at all. Set by `Plot::set_crosshair()`; cleared by
   /// `Plot::clear_crosshair()`, which `input::Controller` calls when the pointer leaves the
@@ -68,6 +96,10 @@ struct Crosshair {
   /// The data y of the horizontal line, on y axis 0. Meaningless for a vertical-only
   /// crosshair, which draws no row and anchors its readout at the top of the axes.
   f64 y = 0.0;
+  /// Whether a row's value is one of the series' samples or a blend of two -- see
+  /// `CrosshairValue`, which also says where each mode puts the marker. Set through
+  /// `Plot::set_crosshair_value_mode()`.
+  CrosshairValue value_mode = CrosshairValue::NearestSample;
   /// This plot's own stroke and dash, when it wants to differ from the global style; a
   /// default-constructed `CrosshairStyle` defers every field to `render::Style` (see
   /// `CrosshairStyle`). Set through `Plot::set_crosshair_style()`.
@@ -85,19 +117,17 @@ struct Crosshair {
 /// arithmetic, which is O(1) in the series' length. The crosshair runs on every pointer
 /// move, so that is a requirement rather than a nicety.
 ///
-/// **`t` chooses between the two samples; it does not blend them.** The value a caller
+/// **`t` is a position, not yet an answer.** It says where the query sits between the two
-/// builds from this span is the y of whichever end `t` is nearer to (`Plot::crosshair_value()`
+/// samples and nothing more; what a caller makes of it is `CrosshairValue`'s to decide. In
-/// takes `t <= 0.5` as `begin`), so a readout quotes a sample the series actually holds. It
+/// `NearestSample` -- the default -- it *selects*, `t <= 0.5` taking `begin`, so the readout
-/// used to be a linear interpolation between the two, which reads as "the signal at this
+/// quotes a sample the series actually holds. In `Interpolated` it *blends*, through
-/// instant" -- a number that is *between* the data and therefore in no row of it. On a
+/// `crosshair_interpolate()`. A plot cannot tell which is right, because the question is
-/// decimated or averaged source that is a value nothing measured, and at a zoom past the
+/// about the data and not about the drawing; `CrosshairValue` is where that is argued.
-/// sample spacing it is visibly not the datum the pointer is over.
 ///
-/// So `t` is a *selector*, and this remains a different convention from
+/// Either way this is a different convention from `Series::reduce()`, `sum()` and `sumsq()`,
-/// `Series::reduce()`, `sum()` and `sumsq()`, which are **exact over a sample range**: those
+/// which are **exact over a sample range**: those answer "what are the samples'
-/// answer "what are the samples' extremes/total over this interval", this answers "which
+/// extremes/total over this interval", a crosshair answers "what is the signal at this one
-/// sample is under the pointer, and what is its value". Both now quote the data; what differs
+/// x". What differs is a statistic of many samples against a reading at one place.
-/// is whether the answer is one sample or a statistic of many.
 ///
 /// `valid` is false -- and every field left at its default -- for an **empty series**, a
 /// non-positive `dx()`, a non-finite `x`, and an `x` strictly outside the series' window
@@ -110,13 +140,13 @@ struct CrosshairSpan {
   /// Index of the first sample whose x is at or after the query: `begin` when the query
   /// lands on a sample, `begin + 1` between two of them.
   u64 end = 0;
-  /// The query's position between the two samples, in `[0, 1]`. A *selector*, not a blend
+  /// The query's position between the two samples, in `[0, 1]`. Whether it selects one of
-  /// factor -- see the struct comment.
+  /// them or blends both is `CrosshairValue`'s -- see the struct comment.
   f64 t = 0.0;
   bool valid = false;
 };
 
-/// The sample span `x` falls in, on `series`; see `CrosshairSpan` for the rule, for the
+/// The sample span `x` falls in, on `series`; see `CrosshairSpan` for what `t` means, for the
 /// convention it is half of, and for the cases that yield `valid == false`.
 ///
 /// O(1) and allocation-free: a subtraction, a division and a truncation. The series' own sample
@@ -125,6 +155,15 @@ struct CrosshairSpan {
 /// allocates.
 CrosshairSpan crosshair_span(const data::Series& series, f64 x) noexcept;
 
+/// The linear blend of a span's two end samples at `t`: `y_begin + t * (y_end - y_begin)`,
+/// which is what `CrosshairValue::Interpolated` reports.
+///
+/// Written as that form rather than `(1 - t) * a + t * b` so `t == 0` returns `y_begin`
+/// exactly and `t == 1` returns `y_end` exactly, with no rounding of its own at the ends: a
+/// query that landed on a sample must read that sample's value back unchanged, whichever mode
+/// is in force. `NearestSample` does not call it at all.
+f64 crosshair_interpolate(f64 y_begin, f64 y_end, f64 t) noexcept;
+
 /// One trace's line of the readout: which trace it is about, the value under the pointer
 /// and the boxes the renderer stages.
 ///
@@ -150,8 +189,14 @@ struct CrosshairRow {
   /// entries -- so a row that is not listed is still a listed *trace*, with a marker, just
   /// not a line in a panel that has no room for it.
   bool listed = false;
-  /// The y of the trace's sample nearest the crosshair's x, on the trace's own y axis. Only
+  /// The data x the row's value belongs to, on the trace's own x axis: in
-  /// meaningful when `valid`.
+  /// `CrosshairValue::NearestSample` the **quoted sample's own x**, which is generally not the
+  /// crosshair's; in `Interpolated` the crosshair's x itself, since that is where an
+  /// interpolated number is. `marker` is placed from this and from `y`. Only meaningful when
+  /// `valid`.
+  f64 x = 0.0;
+  /// The row's value, on the trace's own y axis: the nearer sample's y, or the blend of the
+  /// two -- see `CrosshairValue`. Only meaningful when `valid`.
   f64 y = 0.0;
   /// The trace's name, or the positional placeholder of an unnamed one.
   std::string label;
@@ -161,12 +206,24 @@ struct CrosshairRow {
   /// `kCrosshairNoValue`, the em dash. The absence is therefore *legible* -- a blank cell reads as
   /// "still loading" rather than as "nothing here" -- and nothing numeric is fabricated for it.
   std::string value;
-  /// Where the renderer draws the marker, in framebuffer pixels: the pen for `y`, placed
+  /// Where the renderer draws the marker, in framebuffer pixels: the pen for `(x, y)`, placed
   /// through the *trace's own* axis pair, so a trace on a secondary y axis is marked on the
-  /// scale it is drawn against. It is the nearest **sample**, so at a zoom past the sample
+  /// scale it is drawn against.
-  /// spacing the marker sits beside the polyline rather than on it -- deliberately: it marks
+  ///
-  /// the datum the row reports, and one y feeding both keeps the dot and the number from ever
+  /// It marks **the point the row reports**, which is the whole job: in
-  /// disagreeing. Meaningless when `!valid`.
+  /// `CrosshairValue::NearestSample` it is *snapped to the quoted sample*, sitting on the datum
+  /// rather than beside it on the crosshair's column, so the mark says which of the samples
+  /// under the pointer the number came from. (Placing it at the pointer's x with the sample's y
+  /// put it off the data in both coordinates at a deep zoom -- a mark on nothing.) In
+  /// `Interpolated` it stays on the crosshair's column, on the polyline, which is equally where
+  /// *that* mode's number is. One `(x, y)` feeding both the mark and the text is what keeps them
+  /// from ever disagreeing.
+  ///
+  /// It is drawn `Style::crosshair_marker_px` across -- **bigger than a trace's own sample
+  /// markers** on purpose, since a snapped mark lands among them and has to read as "this one"
+  /// rather than as one more sample. A mark for a sample outside the visible range is staged
+  /// where that sample is and left to the axes clip, rather than being dropped by a range test
+  /// of the drawing layer's own. Meaningless when `!valid`.
   Vec2 marker{};
   /// The swatch's box, `Legend::swatch_w` x `Legend::swatch_h` centred in the row. Empty
   /// when `!listed`.

include/plot/plot/figure.hpp

@@ -138,6 +138,16 @@ class Figure {
   }
   f32 min_y_gutter() const noexcept { return min_y_gutter_; }
 
+  /// The empty band every row keeps to the right of its `axes`, in device pixels (0.0
+  /// by default, so `axes` reaches the viewport's right edge). It is set here rather
+  /// 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.
+  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
   /// and one shared axis is drawn under the last row (the default), or every row
   /// draws its own labels and the figure draws nothing extra.
@@ -326,6 +336,7 @@ class Figure {
   FigureLayout layout_{};
   f32 row_gap_ = 0.0f;
   f32 min_y_gutter_ = 0.0f;
+  f32 right_gutter_ = 0.0f;
   bool labels_bottom_only_ = true;
 };
 

include/plot/plot/plot.hpp

@@ -88,6 +88,11 @@ struct PlotLayout {
   /// no text backend there is no title to lay out at all and both stay 0.0f.
   f32 y_title_width = 0.0f;
   f32 x_title_height = 0.0f;
+  /// The empty band kept between `axes`' right edge and the viewport's, 0.0f unless a
+  /// caller asked for one (see `Plot::set_right_gutter()`). Reported because it is
+  /// clamped by the same cap as the strips above, so what was asked for and what was
+  /// reserved are not always the same number.
+  f32 right_gutter = 0.0f;
   /// The measured legend: inside `axes`, never reserving any part of it, and empty
   /// when there is nothing to draw. It is measured with the same text backend the
   /// strips above are, and against the same `axes` rect the caller reads back, so
@@ -508,6 +513,18 @@ class Plot {
   void set_x_axis_origin_from_range(u32 axis, bool on);
   void set_y_axis_origin_from_range(u32 axis, bool on);
 
+  /// A one-line caption for the x axis, drawn as **chrome** in the bottom-left corner of the
+  /// axes with the legend's own backplate behind it. It exists for the thing a shifted origin
+  /// leaves unsaid: labels counted from a window's left edge are only readable beside the value
+  /// that edge has, and the reader adds the two. An `Annotation` cannot serve -- its anchor is a
+  /// data coordinate, so the caption crawls across the plot under a pan and drifts under a zoom,
+  /// which is the one thing a legend-like panel must not do. The text is the host's, because only
+  /// the host knows how much precision the sum needs: the axis' own label form carries three
+  /// significant figures, which is exactly what made the shifted origin necessary.
+  /// Empty (the default) draws nothing.
+  void set_x_note(std::string text);
+  const std::string& x_note() const noexcept { return x_note_; }
+
   /// The measurement for `id`. Re-evaluated only when a cursor moved, when the
   /// measure was added, or when the series underneath it changed: a different
   /// series, a longer one, or a sliding window whose sample count stayed the same
@@ -628,6 +645,20 @@ class Plot {
   }
   f32 min_y_gutter() const noexcept { return min_y_gutter_; }
 
+  /// An empty band reserved between `axes`' right edge and the viewport's, in device
+  /// pixels. 0.0 -- the default -- is what the layout has always done: `axes` reaches
+  /// the viewport's right edge, which leaves the rightmost x tick label, *centred* on
+  /// that edge, half of it to be drawn in and nothing to draw it in -- and puts the
+  /// newest sample's ink on the frame. It is the mirror of the left gutter in purpose
+  /// and not in kind: nothing is drawn here, so there is nothing to measure and the
+  /// value is the caller's to choose. Reserved inside the same cap as the label and
+  /// title strips, so it can no more squeeze `axes` below the half-viewport floor than
+  /// they can. Negative values are clamped to 0.
+  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_; }
+
   /// Grid segments for the primary axis pair (x axis 0, y axis 0), in data space.
   const Grid& grid() const noexcept { return grid_; }
 
@@ -688,23 +719,31 @@ class Plot {
   /// dash, which live on `CrosshairLayout` because they depend on `ui_scale`.
   const CrosshairStyle& crosshair_style() const noexcept { return crosshair_.style; }
 
+  /// Chooses what a readout's numbers *are*: one of the series' own samples, or a value
+  /// computed between two of them. `CrosshairValue` argues the two cases and says where each
+  /// puts the marker; the default is `NearestSample`.
+  ///
+  /// Deliberately **not** part of `set_crosshair()`, like the readout switch and the style
+  /// override: it is a statement about the data, so a pointer move must not be able to change
+  /// it. It takes effect on the next `layout()` and reaches `crosshair_value()` immediately.
+  void set_crosshair_value_mode(CrosshairValue mode) noexcept { crosshair_.value_mode = mode; }
+  /// The readout's value mode, as set -- see `set_crosshair_value_mode()`.
+  CrosshairValue crosshair_value_mode() const noexcept { return crosshair_.value_mode; }
+
   /// The crosshair's state: where it is, which lines it draws and whether the readout is
   /// on. Read-only -- `set_crosshair()` and `clear_crosshair()` are how it moves -- so the
   /// coordinates it holds and the geometry `layout()` places from them cannot disagree.
   const Crosshair& crosshair() const noexcept { return crosshair_; }
 
-  /// The value of `trace`'s **nearest sample** to a data `x`: `crosshair_span()` brackets
+  /// The value this plot's readout would report for `trace` at a data `x`: the nearer
-  /// `x`, and the nearer of the two samples is the answer (`t <= 0.5` takes the earlier) --
+  /// bracketing sample's y, or the blend of the two, according to
-  /// the same lookup the readout's rows are built from, exposed for a host that wants the
+  /// `crosshair_value_mode()` -- the same lookup the readout's rows are built from, exposed
-  /// number for itself (a status line, a tooltip of its own).
+  /// for a host that wants the number for itself (a status line, a tooltip of its own).
-  ///
-  /// Never a value interpolated between two samples: see `CrosshairSpan` for why a readout
-  /// quotes the data rather than the signal it implies.
   ///
   /// False, leaving `out_y` untouched, for an id that names no trace, a trace with no
   /// series, and an `x` the series has no sample at -- the cases `CrosshairSpan::valid`
-  /// documents, plus the trace itself. Never a clamped or nearest-sample answer: see
+  /// documents, plus the trace itself. Never a clamped answer: see `crosshair_span()` for why
-  /// `crosshair_span()` for why an out-of-window x is not a value.
+  /// an out-of-window x is not a value.
   ///
   /// O(1) and allocation-free once the plot has looked a value up before: the span is
   /// index arithmetic and the samples are read through one `Series::gather()` of at most
@@ -712,6 +751,17 @@ class Plot {
   /// on every pointer move -- once per visible trace, so nothing here may scan a series.
   bool crosshair_value(u32 trace, f64 x, f64& out_y) const;
 
+  /// The whole **point** the readout reports, x as well as y: `crosshair_value()`'s number
+  /// together with the data x it belongs to. In `CrosshairValue::NearestSample` that x is the
+  /// quoted sample's own, which is generally not the `x` asked for; in `Interpolated` it is
+  /// `x` itself.
+  ///
+  /// This is what places `CrosshairRow::marker`, and it is public because a host that draws
+  /// instruments of its own -- a cursor pinned to the value it reads, an annotation naming the
+  /// sample -- needs the same snap, and recomputing it would be a second rule that can drift
+  /// from this one. Same failure cases, same cost, same scratch buffer as `crosshair_value()`.
+  bool crosshair_point(u32 trace, f64 x, f64& out_x, f64& out_y) const;
+
  private:
   /// One measure plus its cache. The dirty flag is what keeps measurement() from
   /// re-evaluating every frame; `last_series`, `last_size` and `last_x0` catch a
@@ -829,11 +879,16 @@ class Plot {
   render::Style style_{};
   const render::TextBackend* text_ = nullptr;
   Legend legend_{};
+  /// The x axis' corner caption; see set_x_note(). Empty draws nothing.
+  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;
   /// 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;
+  /// The empty band kept to the right of `axes`; see set_right_gutter(). 0.0 means none,
+  /// which is what the layout did before the setter existed.
+  f32 right_gutter_ = 0.0f;
   /// Which handles a pointer is on, published by the input layer (see HotHandles).
   HotHandles hot_{};
 

include/plot/render/renderer.hpp

@@ -445,11 +445,12 @@ class Renderer {
   /// `origin_x` back in, in f64, before the single f32 narrowing (B1), so a
   /// large-magnitude x -- an epoch axis -- never loses sub-pixel precision.
   void stage_segment(f64 origin_x, f64 x0, f64 y0, f64 x1, f64 y1, bool log_x, bool log_y);
-  /// Stages `n` interleaved `x,y` points of a gathered window (see
+  /// Stages `n` interleaved `x,y` points of a gathered window (see `Series::gather`): x
-  /// `Series::gather`): x **relative to the series' `x0()`**, y absolute. The
+  /// **relative to `origin_x`**, which the caller asked the gather for, y absolute. On linear
-  /// absolute data x is reconstructed in f64 (`series_x0 + rel`) and staged as
+  /// axes that is already what the batch wants and the buffer is handed over as it is; a log
+  /// axis reconstructs the absolute x in f64 per segment and stages
   /// `to_space(x_abs) - to_space(origin_x)` -- O(visible pixels), not O(epoch).
-  void stage_xy(const f32* xy, u32 n, f64 series_x0, f64 origin_x, bool log_x, bool log_y);
+  void stage_xy(const f32* xy, u32 n, f64 origin_x, bool log_x, bool log_y);
   /// Lays one run of text out at a framebuffer position and stages it into
   /// `batch`, shifting it into viewport space and tinting it `color` (the
   /// per-instance `a_colour`). Returns the pen past the run.
@@ -534,6 +535,12 @@ class Renderer {
   /// them again, so the panel a test asserts and the panel this draws are the same
   /// numbers.
   void draw_legend(const Plot& plot, const PlotLayout& layout);
+  /// `Plot::x_note()`, in the bottom-left corner of the axes on the legend's backplate. It is
+  /// measured here rather than in `Plot::layout()` on purpose: unlike a legend it reserves no
+  /// space and nothing can be picked on it, so there is no measurement a host or a test needs to
+  /// read back, and a caption that explains the x labels must not be able to move the layout the
+  /// labels were measured into.
+  void draw_x_note(const Plot& plot, const PlotLayout& layout);
   /// The backplate the legend and the crosshair's readout share: the frame's background at
   /// `kLegendPanelAlpha` over `panel`, with a one-device-pixel border in `border` around the
   /// outermost pixels the fill covers (the crisp-hairline rule -- a border drawn on the measured

include/plot/render/style.hpp

@@ -86,6 +86,21 @@ struct Style {
   f32 crosshair_width = 0.75f;
   f32 crosshair_dash_on = 4.0f;
   f32 crosshair_dash_off = 4.0f;
+  /// The full extent, in logical pixels, of the little cross drawn at each trace's readout
+  /// point (`CrosshairRow::marker`), in that trace's own colour.
+  ///
+  /// **Deliberately larger than `marker_px`**, which is what a trace's own samples are dotted
+  /// with at a deep zoom. In the default `CrosshairValue::NearestSample` the readout's mark is
+  /// *snapped to a sample*, so it lands among those dots -- at the same size it would read as
+  /// one more sample instead of as "this is the one the panel is quoting". The lines are chrome
+  /// and recede; this mark is the answer to the question the hover asks, and it is the one
+  /// crosshair element that should not.
+  ///
+  /// Its stroke is `crosshair_marker_width`, thicker than the lines' for the same reason. `0`
+  /// across draws no mark at all, which is how a caller that wants the panel without the marks
+  /// says so.
+  f32 crosshair_marker_px = 9.0f;
+  f32 crosshair_marker_width = 1.5f;
 
   // --- the scale ----------------------------------------------------------
 

src/data/array_series.cpp

@@ -75,9 +75,9 @@ Range ArraySeries::y_bounds() const { return y_bounds_; }
 
 const MinMaxPyramid& ArraySeries::pyramid() const { return pyramid_; }
 
-void ArraySeries::gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+void ArraySeries::gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                          GatherMode mode) const {
-  gather_impl(level, begin, end, out_xy, mode);
+  gather_impl(level, begin, end, x_origin, out_xy, mode);
 }
 
 void ArraySeries::copy_raw(u64 begin, u64 end, f32* out) const {

src/data/series.cpp

@@ -176,21 +176,24 @@ u64 Series::snap_index(f64 x) const {
   return static_cast<u64>(k);
 }
 
-void Series::gather_raw(u64 begin, u64 end, std::vector<f32>& out_xy) const {
+void Series::gather_raw(u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy) const {
   const f64 step = dx();
+  // Sample i sits at `x0() + i*step`, so this is `x_at(i) - x_origin` with the subtraction done
+  // once in f64 rather than per sample: see gather() for why the caller owns the origin.
+  const f64 shift = x0() - x_origin;
   f32 raw[kGatherChunk];
   for (u64 i = begin; i < end;) {
     const u64 take = std::min<u64>(kGatherChunk, end - i);
     copy_raw(i, i + take, raw);
     for (u64 k = 0; k < take; ++k) {
-      out_xy.push_back(f32(f64(i + k) * step));   // x relative to x0(): see gather()
+      out_xy.push_back(f32(shift + f64(i + k) * step));   // x relative to x_origin: see gather()
       out_xy.push_back(raw[k]);
     }
     i += take;
   }
 }
 
-void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+void Series::gather_impl(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                          GatherMode mode) const {
   const u64 n = size();
   const u64 b = std::min(begin, n);
@@ -198,7 +201,7 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
   if (b >= e) return;
 
   if (level == 0) {
-    gather_raw(b, e, out_xy);
+    gather_raw(b, e, x_origin, out_xy);
     return;
   }
   // Public levels clamp to the deepest representable one (R3): past
@@ -226,6 +229,7 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
   // picks, so the envelope path stays O(buckets) and the geometry still spans
   // the requested range exactly, for every mode.
   const f64 step = dx();
+  const f64 shift = x0() - x_origin;   // as in gather_raw(): x_at(i) - x_origin = shift + i*step
   const u64 span = u64{kBucket} << level;
   const u64 origin = pyramid().window_base();
   const u64 lbase = pyramid().level_base(level);
@@ -235,11 +239,11 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
   if (first >= last) {
     // No complete bucket of this level is inside the range -- a window squeezed
     // into one bucket -- so the raw samples are the only exact geometry there is.
-    gather_raw(b, e, out_xy);
+    gather_raw(b, e, x_origin, out_xy);
     return;
   }
   const u64 lead = first * span - origin;   // window index the first emitted bucket starts at
-  if (b < lead) gather_raw(b, std::min(lead, e), out_xy);
+  if (b < lead) gather_raw(b, std::min(lead, e), x_origin, out_xy);
 
   switch (mode) {
     case GatherMode::Envelope: {
@@ -252,9 +256,9 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
         const MinMax m = pyramid().bucket(level, bi);
         const u64 first_sample = bi * span - origin;   // >= 0: stored buckets start inside the window
         const u64 last_sample = std::min((bi + 1) * span, origin + e) - 1 - origin;
-        out_xy.push_back(f32(f64(first_sample) * step));   // x relative to x0()
+        out_xy.push_back(f32(shift + f64(first_sample) * step));   // x relative to x_origin
         out_xy.push_back(m.min);
-        out_xy.push_back(f32(f64(last_sample) * step));
+        out_xy.push_back(f32(shift + f64(last_sample) * step));
         out_xy.push_back(m.max);
       }
       break;
@@ -269,7 +273,7 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
         const u64 centre = bi * span + span / 2 - origin;   // inside [b, e): the bucket is wholly inside
         f32 v = 0.0f;
         copy_raw(centre, centre + 1, &v);
-        out_xy.push_back(f32(f64(centre) * step));   // x relative to x0()
+        out_xy.push_back(f32(shift + f64(centre) * step));   // x relative to x_origin
         out_xy.push_back(v);
       }
       break;
@@ -283,7 +287,7 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
         const u32 count = pyramid().bucket_count(level, bi);
         if (count == 0) continue;
         const u64 centre = bi * span + span / 2 - origin;
-        out_xy.push_back(f32(f64(centre) * step));   // x relative to x0()
+        out_xy.push_back(f32(shift + f64(centre) * step));   // x relative to x_origin
         out_xy.push_back(f32(pyramid().bucket_sum(level, bi) / f64(count)));
       }
       break;
@@ -291,7 +295,7 @@ void Series::gather_impl(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy
   }
 
   const u64 trail = last * span - origin;   // one past the last emitted bucket's span
-  if (trail < e) gather_raw(trail, e, out_xy);
+  if (trail < e) gather_raw(trail, e, x_origin, out_xy);
 }
 
 }  // namespace plot::data

src/data/stream_series.cpp

@@ -87,9 +87,9 @@ Range StreamSeries::y_bounds() const { return y_bounds_; }
 
 const MinMaxPyramid& StreamSeries::pyramid() const { return pyramid_; }
 
-void StreamSeries::gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+void StreamSeries::gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                           GatherMode mode) const {
-  gather_impl(level, begin, end, out_xy, mode);
+  gather_impl(level, begin, end, x_origin, out_xy, mode);
 }
 
 void StreamSeries::copy_raw(u64 begin, u64 end, f32* out) const {

src/data/xy_series.cpp

@@ -100,16 +100,16 @@ u64 XYSeries::snap_index(f64 x) const {
              : hi;                      // ties go to the lower index
 }
 
-void XYSeries::gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
+void XYSeries::gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out_xy,
                       GatherMode mode) const {
   const u64 n = size();
   const u64 b = std::min(begin, n);
   const u64 e = std::min(std::max(end, b), n);
   if (b >= e) return;
 
-  // x emitted relative to x0(): see Series::gather(). The x array is f64, so
+  // x emitted relative to the caller's origin: see Series::gather(). The x array is f64, so
   // the subtraction happens wide and only the small, exact offset narrows.
-  const f64 origin = x0();
+  const f64 origin = x_origin;
 
   if (level == 0) {
     for (u64 i = b; i < e; ++i) {
@@ -127,7 +127,7 @@ void XYSeries::gather(u32 level, u64 begin, u64 end, std::vector<f32>& out_xy,
   // `begin`, `last` one past the last bucket wholly before `end`, and the two
   // edge spans around them are read from the raw arrays -- so no emitted
   // geometry comes from outside [begin, end), and the emitted x values are the
-  // array's own coordinates relative to x0().
+  // array's own coordinates relative to the caller's origin.
   const u64 span = u64{kBucket} << level;
   const u64 lend = pyramid().level_size(level);
   const u64 first = std::min(ceil_div(b, span), lend);

src/plot/figure.cpp

@@ -451,6 +451,10 @@ const FigureLayout& Figure::layout() {
     // not drawn once per row. A figure told to label every row keeps them everywhere,
     // and then there is no shared axis to speak of: each row has its own.
     p.set_x_strip_mode(labels_bottom_only_ && row != bottom_row ? XStrip::None : XStrip::Both);
+    // Before either pass below, and the same value for every row: nothing is measured on
+    // the right, so unlike the left gutter this needs no measurement pass -- the rows end
+    // on a common column because they are all told the one width.
+    p.set_right_gutter(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

src/plot/plot.cpp

@@ -342,6 +342,8 @@ void Plot::set_y_axis_origin_from_range(u32 axis, bool on) {
   y_axes_[axis].origin_follows_range = on;
 }
 
+void Plot::set_x_note(std::string text) { x_note_ = std::move(text); }
+
 f64 Plot::x_axis_origin(u32 axis) const {
   return axis < x_axes_.size() ? x_axes_[axis].origin : 0.0;
 }
@@ -1037,12 +1039,21 @@ const PlotLayout& Plot::layout() {
     x_height = std::min(x_height, x_budget);
     const f32 y_title_width = std::min(y_title_w, y_budget - y_width);
     const f32 x_title_height = std::min(x_title_h, x_budget - x_height);
+    // The right-hand band, out of what is left of the same allowance the two left
+    // strips were served from -- so the three together cannot take more than
+    // `1 - kMinAxesShare` of the width however much each asks for. Nothing is drawn
+    // in it, so unlike the strips above it is a want and never a measurement, and it
+    // cannot feed the fixed point below: the y labels are chosen from the row's range
+    // and height, so a narrower `axes` does not change what they need.
+    const f32 right_gutter =
+        std::min(right_gutter_, y_budget - (y_width + y_title_width));
     layout_.y_axis_width = y_width;
     layout_.x_axis_height = x_height;
     layout_.y_title_width = y_title_width;
     layout_.x_title_height = x_title_height;
+    layout_.right_gutter = right_gutter;
     layout_.axes = Rect{viewport_.x + y_width + y_title_width, viewport_.y,
-                        vw - (y_width + y_title_width),
+                        vw - (y_width + y_title_width + right_gutter),
                         vh - (x_height + x_title_height)};
     layout_.y_title_area = Rect{viewport_.x, layout_.axes.y, y_title_width, layout_.axes.h};
     layout_.y_label_area =
@@ -1410,6 +1421,13 @@ CrosshairSpan crosshair_span(const data::Series& series, f64 x) noexcept {
   return span;
 }
 
+f64 crosshair_interpolate(f64 y_begin, f64 y_end, f64 t) noexcept {
+  // This form, not `(1 - t) * a + t * b`: at `t == 0` it is `y_begin` and at `t == 1` it is
+  // `y_end`, both exactly, so a query that landed on a sample reads that sample back whatever
+  // mode is in force. The symmetric form rounds at both ends.
+  return y_begin + t * (y_end - y_begin);
+}
+
 std::string crosshair_unnamed_label(u32 trace) {
   char buf[16];
   std::snprintf(buf, sizeof buf, "#%u", static_cast<unsigned>(trace));
@@ -1439,6 +1457,11 @@ void Plot::set_crosshair(f64 x, f64 y, bool active) {
 }
 
 bool Plot::crosshair_value(u32 trace, f64 x, f64& out_y) const {
+  f64 ignored = 0.0;
+  return crosshair_point(trace, x, ignored, out_y);
+}
+
+bool Plot::crosshair_point(u32 trace, f64 x, f64& out_x, f64& out_y) const {
   if (trace >= traces_.size()) return false;
   const data::Series* series = traces_[trace].series;
   if (series == nullptr) return false;
@@ -1450,17 +1473,31 @@ bool Plot::crosshair_value(u32 trace, f64 x, f64& out_y) const {
   // when it fell between them. The scratch is cleared and refilled rather than reallocated,
   // and sized for two pairs from construction, so the pointer path allocates nothing.
   crosshair_scratch_.clear();
-  series->gather(0u, span.begin, span.end + 1u, crosshair_scratch_);
+  // The origin is the query itself: only y is read out of the pairs below -- the x comes back
+  // through `x_at()` -- so this names the one origin that cannot lose precision for a caller
+  // that might.
+  series->gather(0u, span.begin, span.end + 1u, x, crosshair_scratch_);
   const usize pairs = static_cast<usize>(span.end - span.begin + 1u);
   if (crosshair_scratch_.size() != 2u * pairs) return false;  // a series that produced nothing
   const f64 y_begin = static_cast<f64>(crosshair_scratch_[1u]);
   const f64 y_end = static_cast<f64>(crosshair_scratch_[2u * pairs - 1u]);
-  // **The nearer sample, never a value between two of them.** A readout quotes the data: a
+  if (crosshair_.value_mode == CrosshairValue::Interpolated) {
-  // number the series does not contain is a claim about a signal between its samples, which
+    // The signal the samples imply *at* this x, so the point is the query's own x: that is
-  // is the caller's to make and not a plot's to put in a panel. `span.t` is measured on the
+    // where an interpolated number lives, and the marker on the polyline there is honest
-  // samples' own x, so "nearer" is exact on any grid; a tie reads the earlier sample, which
+    // about it.
-  // is the bias `crosshair_span()` already takes for a degenerate pair.
+    out_x = x;
-  out_y = span.t <= 0.5 ? y_begin : y_end;
+    out_y = crosshair_interpolate(y_begin, y_end, span.t);
+    return true;
+  }
+  // `NearestSample`: the nearer of the two, and **its own x with it**. `span.t` is measured on
+  // the samples' own coordinates, so "nearer" is exact on any grid; a tie reads the earlier
+  // sample, which is the bias `crosshair_span()` already takes for a degenerate pair. The x
+  // comes back through `x_at()` rather than from the gathered pairs, which are relative to a
+  // caller-named origin by the `Series::gather()` contract -- adding it back here would be a
+  // second reconstruction of a coordinate the series will hand over directly.
+  const bool take_begin = span.t <= 0.5;
+  out_x = series->x_at(take_begin ? span.begin : span.end);
+  out_y = take_begin ? y_begin : y_end;
   return true;
 }
 
@@ -1529,17 +1566,23 @@ void Plot::build_crosshair() {
     row.trace = i;
     row.valid = false;
     row.listed = false;
+    row.x = 0.0;
     row.y = 0.0;
     row.marker = Vec2{};
     row.swatch = Rect{};
     row.label_box = Rect{};
     row.value_box = Rect{};
     row.label.assign(trace.name.empty() ? crosshair_unnamed_label(i) : trace.name);
-    // The value: the nearest sample's number where the trace has one at this x, and the em dash
+    // The value where the trace has one at this x, and the em dash where it does not. The row is
-    // where it does not. The row is listed either way, so the absence has to be *said* -- a blank
+    // listed either way, so the absence has to be *said* -- a blank value cell reads as a value
-    // value cell reads as a value that has not arrived -- and the dash is measured into the panel's
+    // that has not arrived -- and the dash is measured into the panel's width like any other
-    // width like any other value, so the row's boxes are placed by one rule.
+    // value, so the row's boxes are placed by one rule.
-    if (crosshair_value(i, crosshair_.x, row.y)) {
+    //
+    // The **point**, not just the number: `crosshair_point()` reports the x the value belongs to
+    // as well, which in the default `NearestSample` mode is the quoted sample's own and not the
+    // pointer's. Both coordinates come from that one call, so the mark and the text cannot
+    // disagree about which datum is being read (see `CrosshairRow::marker`).
+    if (crosshair_point(i, crosshair_.x, row.x, row.y)) {
       row.valid = true;
       char value[32];
       // The trace's *own* y axis formats the number, so a readout reads exactly like the
@@ -1548,7 +1591,7 @@ void Plot::build_crosshair() {
       row.value.assign(value);
       // Placed through the trace's own axis pair, so the marker sits where that trace is
       // actually drawn however its axes are scaled.
-      row.marker = Vec2{static_cast<f32>(x_transform(trace.axis_x).to_screen_x(crosshair_.x)),
+      row.marker = Vec2{static_cast<f32>(x_transform(trace.axis_x).to_screen_x(row.x)),
                         static_cast<f32>(y_transform(trace.axis_y).to_screen_y(row.y))};
     } else {
       row.value.assign(kCrosshairNoValue);

src/plot/ticks.cpp

@@ -98,7 +98,10 @@ void format_time(f64 seconds, char* buf, usize n) {
   const char* const sign = seconds < 0.0 ? "-" : "";
   const f64 t = std::fabs(seconds);
 
-  if (t < 1.0) {
+  // `0.9995` and not `1.0`: three significant figures of 0.9999 s are `1e+03ms`, so a value
+  // that *rounds* to a second belongs to the whole-second branch below, which writes it `1s`.
+  // The same spill one rung down is handled inside the ladder.
+  if (t < 0.9995) {
     // Sub-second: the prefix that keeps the mantissa in [1, 1000) -- the same rule the SI
     // formatter uses on a y axis, and the reading a zoomed-in window needs. A fixed
     // millisecond unit printed `0.005ms` where `5us` is what the axis is showing, and
@@ -119,8 +122,12 @@ void format_time(f64 seconds, char* buf, usize n) {
         {1.0e-9, "ns"},
         {1.0e-12, "ps"},
     };
+    // The rung is picked by the *rounded* mantissa, which is what `format_si`'s own 999.5
+    // check buys there: 0.9999 ms is `999.9µs` before rounding and `1e+03µs` after, so it is
+    // the millisecond rung's value and reads `1ms`. Testing the raw `t < u.scale` let the
+    // mantissa reach 1000 at every rung boundary.
     for (const auto& u : kUnits) {
-      if (t < u.scale) continue;
+      if (t < u.scale * 0.9995) continue;
       std::snprintf(buf, n, "%s%.3g%s", sign, t / u.scale, u.unit);
       return;
     }

src/render/primitives.cpp

@@ -9,12 +9,17 @@
 namespace plot::render {
 namespace {
 
+#ifndef NDEBUG
 /// Two declared cells agree when neither size differs by more than the atlas'
 /// tolerance: the test `glyph_is_cell()` makes, for a cell against a cell.
+///
+/// Guarded because its one caller is an `assert` (see `TextBatch::set_cell`), so a
+/// release build has no use for it and would warn that it is unused.
 bool same_cell(Vec2 a, Vec2 b) noexcept {
   const auto near = [](f32 p, f32 q) { return std::fabs(p - q) <= kAtlasCellTolerance; };
   return near(a.x, b.x) && near(a.y, b.y);
 }
+#endif
 
 /// `axis` as a unit vector: the cell's local +x, which is what the instance and
 /// the shader both take (see `TextInstance`). A zero, infinite or NaN direction

src/render/renderer.cpp

@@ -533,6 +533,7 @@ void Renderer::draw_plot(Plot& plot, const PlotLayout& layout) {
   draw_measures(plot, layout);
   draw_annotations(plot);
   draw_legend(plot, layout);
+  draw_x_note(plot, layout);
   // The under text batch goes out here, so the readout's panel below it can
   // occlude what it covers: legend names, cursor labels, measure labels and
   // annotation captions are all chrome the panel is allowed to sit on.
@@ -735,24 +736,16 @@ void Renderer::stage_segment(f64 origin_x, f64 x0, f64 y0, f64 x1, f64 y1, bool
                           static_cast<f32>(space(y1, log_y))});
 }
 
-void Renderer::stage_xy(const f32* xy, u32 n, f64 series_x0, f64 origin_x, bool log_x,
+void Renderer::stage_xy(const f32* xy, u32 n, f64 origin_x, bool log_x, bool log_y) {
-                        bool log_y) {
   if (!log_x && !log_y) {
-    // The common case: reconstruct the absolute data x in f64 (`x0 + rel` --
+    // The common case costs nothing at all: the gather emitted x relative to this very origin
-    // the gather contract's own reconstruction), stage it relative to the
+    // (`draw_traces` asks it for `x_range.min`, which is the draw's origin too) and a linear y
-    // draw's origin, and the f32 is O(visible pixels), not O(epoch).
+    // needs no mapping, so the gathered buffer *is* the staged one.
-    staged_.clear();
+    lines_.add_polyline_xy(xy, n);
-    staged_.reserve(usize(n) * 2u);
-    const f64 origin_s = space(origin_x, log_x);
-    for (u32 i = 0; i < n; ++i) {
-      staged_.push_back(static_cast<f32>(xy[2u * i] + series_x0 - origin_s));
-      staged_.push_back(xy[2u * i + 1u]);
-    }
-    lines_.add_polyline_xy(staged_.data(), n);
     return;
   }
   for (u32 i = 0; i + 1u < n; ++i) {
-    stage_segment(origin_x, xy[2u * i] + series_x0, xy[2u * i + 1u], xy[2u * i + 2u] + series_x0,
+    stage_segment(origin_x, xy[2u * i] + origin_x, xy[2u * i + 1u], xy[2u * i + 2u] + origin_x,
                   xy[2u * i + 3u], log_x, log_y);
   }
 }
@@ -1025,7 +1018,12 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
                           : 0u;
 
     gather_.clear();
-    series.gather(level, first, last, gather_, trace.mode);
+    // The visible range's minimum is the gather origin: the emitted f32 is then O(the span on
+    // screen), so its ULP is a fixed 2^-24 of the axis at every zoom. `series.x0()` -- the origin
+    // before -- is the start of the *retained window*, so the quantisation was a fraction of data
+    // the view need not even hold, and a deep zoom drew a sample visibly off the x a readout
+    // quotes for it. See `Series::gather()`.
+    series.gather(level, first, last, x_range.min, gather_, trace.mode);
     if (gather_.size() < 4u) continue;  // fewer than two points is no segment
 
     stats_.visible_samples += visible;
@@ -1040,11 +1038,11 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
       const auto& yt = plot.y_transform(trace.axis_y);
       const f32 half = 0.5f * style_.device(style_.marker_px);
       for (u32 k = 0; k + 1 < gather_.size(); k += 2) {
-        const f64 x_abs = series.x0() + static_cast<f64>(gather_[k]);
+        const f64 x_abs = x_range.min + static_cast<f64>(gather_[k]);
         const f64 y_abs = static_cast<f64>(gather_[k + 1]);
         if (!std::isfinite(x_abs) || !std::isfinite(y_abs)) continue;
-        const f32 sx = xt.to_screen_x(x_abs);
+        const f32 sx = static_cast<f32>(xt.to_screen_x(x_abs));
-        const f32 sy = yt.to_screen_y(y_abs);
+        const f32 sy = static_cast<f32>(yt.to_screen_y(y_abs));
         if (!(sx >= layout.axes.x && sx <= layout.axes.right() && sy >= layout.axes.y &&
               sy <= layout.axes.bottom())) {
           continue;
@@ -1058,11 +1056,11 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
       ++stats_.lod_traces;
     }
 
-    // Gathered x is relative to `series.x0()` (the gather contract); the staging
+    // Gathered x is relative to `x_range.min` (the gather contract, with the origin this
-    // reconstructs the absolute data x in f64 and emits `to_space(x_abs) -
+    // function asked for); the staging reconstructs the absolute data x in f64 and emits
-    // to_space(origin)` -- the visible x range's minimum is the natural origin
+    // `to_space(x_abs) - to_space(origin)` about that same origin, so the f32 is O(visible
-    // for a trace -- so the f32 is O(visible pixels), not O(epoch) (B1). The
+    // pixels), not O(epoch) (B1). The mapping folds the origin back into the offset uniform,
-    // mapping folds the origin back into the offset uniform, in f64.
+    // in f64.
     const bool log_x = plot.x_axis(trace.axis_x).log;
     const bool log_y = plot.y_axis(trace.axis_y).log;
     const Mapping map = mapping(plot, trace.axis_x, trace.axis_y, x_range.min);
@@ -1085,12 +1083,12 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
       usize kept = 0;
       f64 next = std::numeric_limits<f64>::infinity();
       for (usize k = count; k > 0; --k) {
-        const usize i = k - 1u;
+        const usize p = k - 1u;  // the point index; `i` above is the trace's
-        const f64 x = static_cast<f64>(gather_[2u * i]) + series.x0();
+        const f64 x = static_cast<f64>(gather_[2u * p]) + x_range.min;
         if (!(x < next)) continue;  // out of order with what follows: unreadable, and a NaN too
         next = x;
-        gather_[2u * (count - 1u - kept)] = gather_[2u * i];
+        gather_[2u * (count - 1u - kept)] = gather_[2u * p];
-        gather_[2u * (count - 1u - kept) + 1u] = gather_[2u * i + 1u];
+        gather_[2u * (count - 1u - kept) + 1u] = gather_[2u * p + 1u];
         ++kept;
       }
       if (kept != count) {
@@ -1113,7 +1111,7 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
     // trace/axis hierarchy on a high-DPI panel. `width == 0` is scatter: the
     // markers below are the whole trace.
     if (trace.width > 0.0f) {
-      stage_xy(gather_.data(), points, series.x0(), x_range.min, log_x, log_y);
+      stage_xy(gather_.data(), points, x_range.min, log_x, log_y);
       flush_lines(map, trace.color, style_.device(trace.width), dash);
     }
 
@@ -1131,7 +1129,7 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
       const f64 origin_s = space(x_range.min, log_x);
       u32 staged = 0;
       for (u32 p = 0; p < points; p += stride) {
-        const f64 x_abs = static_cast<f64>(gather_[2u * p]) + series.x0();
+        const f64 x_abs = static_cast<f64>(gather_[2u * p]) + x_range.min;
         const f32 rx = static_cast<f32>(space(x_abs, log_x) - origin_s);
         const f32 ry = static_cast<f32>(space(gather_[2u * p + 1u], log_y));
         const Vec2 centre{rx * map.scale.x + map.offset.x, ry * map.scale.y + map.offset.y};
@@ -1403,6 +1401,28 @@ void Renderer::draw_legend(const Plot& plot, const PlotLayout& layout) {
   }
 }
 
+void Renderer::draw_x_note(const Plot& plot, const PlotLayout& layout) {
+  const std::string& text = plot.x_note();
+  if (text.empty()) return;
+  const Rect& axes = layout.axes;
+  const f32 line_h = font_backend_.height();
+  const f32 width = font_backend_.measure(text.c_str());
+  if (!(width > 0.0f) || !(line_h > 0.0f)) return;
+  // The pad is the legend's, so the two panels read as the same kind of chrome.
+  const f32 pad = style_.device(4.0f);
+  const Rect panel{axes.x + pad, axes.bottom() - pad - line_h - 2.0f * pad, width + 2.0f * pad,
+                   line_h + 2.0f * pad};
+  // 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);
+  // 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
+  // it: text staged into the over batch would be drawn before its own backplate.
+  stage_text(glyphs_under_, {panel.x + pad, panel.y + pad}, text.c_str(), style_.axis_text);
+}
+
 void Renderer::stage_marker(plot::Marker marker, Vec2 centre_px, f32 size_px) {
   // Marker shapes as line segments, in viewport pixels: the trace's flush maps
   // them with `pixel_mapping()`, so the size is device pixels whatever the data
@@ -1522,19 +1542,28 @@ void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout)
     flush_lines(data, style_.crosshair, stroke, crosshair.dash);
   }
 
-  // A small cross at each trace's intersection, in the trace's own colour -- opaque, like a
+  // A cross at each trace's readout point, in the trace's own colour -- opaque, like a legend's
-  // legend's swatch, so a trace drawn at 15% still has a marker that can be seen. The arm
+  // swatch, so a trace drawn at 15% still has a mark that can be seen. The position is the
-  // length is a logical size like every other marker's, and the position is the layout's: a
+  // layout's, and in the default `CrosshairValue::NearestSample` that is the *quoted sample*
-  // marker for a sample far outside the visible y range is staged where the trace is and the
+  // rather than the pointer's column: a mark for a sample outside the visible range is staged
-  // axes clip keeps it inside, rather than being skipped by a range test of this layer's own.
+  // where that sample is and the axes clip keeps it inside, rather than being skipped by a range
-  const f32 arm = style_.device(3.0f);
+  // test of this layer's own.
+  //
+  // `crosshair_marker_px` is the cross' full extent and is larger than `marker_px`, which dots a
+  // trace's own samples at a deep zoom -- a snapped mark lands among those dots and has to read
+  // as the one the panel is quoting. `0` across draws nothing, so a caller can have the panel
+  // and the lines without the marks.
+  const f32 arm = 0.5f * style_.device(style_.crosshair_marker_px);
+  const f32 mark_stroke = style_.device(style_.crosshair_marker_width);
+  if (arm > 0.0f) {
     for (const CrosshairRow& row : crosshair.rows) {
       if (!row.valid) continue;
       Color color = plot.trace(row.trace).color;
       color.a = 1.0f;
       lines_.add_segment({row.marker.x - arm, row.marker.y}, {row.marker.x + arm, row.marker.y});
       lines_.add_segment({row.marker.x, row.marker.y - arm}, {row.marker.x, row.marker.y + arm});
-    flush_lines(pixel_mapping(), color, style_.device(1.0f));
+      flush_lines(pixel_mapping(), color, mark_stroke);
+    }
   }
 }
 

tests/test_figure.cpp

@@ -425,6 +425,52 @@ PLOT_TEST(figure_gutter_never_squeezes_the_axes_below_the_floor) {
   }
 }
 
+/// The other side of the same band. `axes` reached the viewport's right edge, which left
+/// the rightmost x tick label -- centred on it -- half outside the plot, so a figure can
+/// ask for an empty band there. One value for every row, like the left gutter, because
+/// the rows' axes have to end on a common column as well as start on one.
+PLOT_TEST(figure_rows_share_one_right_gutter) {
+  plot::Figure figure = different_magnitude_figure();
+  figure.layout();
+  const plot::f32 flush = figure.plot(0u).layout().axes.right();
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.right_gutter()), 0.0, 1e-12);
+  // The default is the layout as it always was: nothing between the data and the frame.
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(flush), static_cast<plot::f64>(kW), 1e-4);
+
+  figure.set_right_gutter(30.0f);
+  figure.layout();
+  const plot::Rect first = figure.plot(0u).layout().axes;
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    const plot::PlotLayout& rl = figure.plot(row).layout();
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(rl.right_gutter), 30.0, 1e-4);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(rl.axes.right()), static_cast<plot::f64>(kW) - 30.0,
+                    1e-4);
+    // The band comes off the right alone: the left gutter the rows agreed on is untouched,
+    // so a host can balance the two sides without the one moving when it sets the other.
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(rl.axes.x), static_cast<plot::f64>(first.x), 1e-4);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(rl.axes.w), static_cast<plot::f64>(first.w), 1e-4);
+    // ... and the x strip follows the data area rather than the viewport, or the shared
+    // axis would be labelled over a band that holds no data.
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(rl.x_label_area.w), static_cast<plot::f64>(rl.axes.w),
+                    1e-4);
+  }
+  // Clamped by the same cap as the strips: a band wider than the whole figure cannot
+  // squeeze `axes` below the floor `Plot::layout()` guarantees.
+  figure.set_right_gutter(10.0f * kW);
+  figure.layout();
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    const plot::Rect& axes = figure.plot(row).layout().axes;
+    PLOT_CHECK(axes.w >= 0.5f * kW - 1e-3f);
+  }
+  // Negative means "none", like the gutter and the row gap, and the data area goes back
+  // to the viewport's edge.
+  figure.set_right_gutter(-5.0f);
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.right_gutter()), 0.0, 1e-12);
+  figure.layout();
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.plot(0u).layout().axes.right()),
+                  static_cast<plot::f64>(flush), 1e-4);
+}
+
 /// The degenerate neighbour: a figure of one row imposes exactly what that row
 /// measured, so it lays out as the plot would on its own -- the gutter cannot move a
 /// row that has no sibling to line up with.

tests/test_plot.cpp

@@ -1939,11 +1939,11 @@ class CountingSeries final : public plot::data::Series {
   }
   plot::Range y_bounds() const override { return {0.0, 99.0}; }
   const plot::data::MinMaxPyramid& pyramid() const override { return pyramid_; }
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out,
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out,
               plot::data::GatherMode mode = plot::data::GatherMode::Envelope) const override {
     ++gather_calls_;
     samples_read_ += end - begin;
-    gather_impl(level, begin, end, out, mode);
+    gather_impl(level, begin, end, x_origin, out, mode);
   }
 
   u64 gather_calls() const noexcept { return gather_calls_; }
@@ -2068,6 +2068,134 @@ PLOT_TEST(crosshair_value_is_the_nearest_sample) {
   PLOT_CHECK(crosshair.rows[0].value == "10");
 }
 
+/// The mark is **snapped to the sample the row quotes**, which is what makes it say *which*
+/// value is being read. Placing it at the pointer's x with the sample's y put it on neither the
+/// data nor the polyline, and the further a reader zoomed in the further off it sat -- a mark on
+/// nothing.
+///
+/// The queries here are all **off** a sample on purpose, so "snapped" has something to be wrong
+/// about: the assertion is that `row.x` is a `tens` grid position (0, 1, 2, 3) and not the query,
+/// and that `row.marker` is that position mapped through the trace's own axes. The pixel column
+/// is computed from the transform the plot already exposes, so this checks the two agree rather
+/// than re-deriving the mapping.
+PLOT_TEST(the_crosshair_marker_snaps_to_the_sample_it_reports) {
+  CrosshairScene scene;
+  const u32 trace = scene.plot.add_trace(
+      {&scene.series, plot::Color::hex(0x40C0FF), 1.5f, scene.ax, scene.ay, "sine"});
+
+  // A query between samples 1 and 2, nearer 2: the point is sample 2 entire -- x = 2 as well as
+  // y = 20 -- so the mark lands on the datum and not a fifth of a division to its left.
+  f64 px = 0.0;
+  f64 py = 0.0;
+  PLOT_CHECK(scene.plot.crosshair_point(trace, 1.8, px, py));
+  PLOT_CHECK_NEAR(px, 2.0, 1e-12);
+  PLOT_CHECK_NEAR(py, 20.0, 1e-9);
+  // The tie takes the earlier sample, and takes *its* x with it: the same bias in both
+  // coordinates, so the mark and the number can never name different samples.
+  PLOT_CHECK(scene.plot.crosshair_point(trace, 1.5, px, py));
+  PLOT_CHECK_NEAR(px, 1.0, 1e-12);
+  PLOT_CHECK_NEAR(py, 10.0, 1e-9);
+  // `crosshair_value()` is the same call with the x dropped, so the two cannot disagree.
+  f64 y_only = 0.0;
+  PLOT_CHECK(scene.plot.crosshair_value(trace, 1.5, y_only));
+  PLOT_CHECK_NEAR(y_only, py, 1e-12);
+  // A failing lookup leaves both outputs alone, as the value form does.
+  PLOT_CHECK(!scene.plot.crosshair_point(trace, 9.0, px, py));
+  PLOT_CHECK_NEAR(px, 1.0, 1e-12);
+
+  // And through the layout: the row carries the snapped x, and the marker is that x through the
+  // trace's own transform -- not the crosshair's column, which is where it used to be.
+  scene.plot.set_crosshair(1.8, true);
+  scene.plot.layout();
+  const plot::CrosshairRow& row = scene.plot.layout().crosshair.rows[0];
+  PLOT_CHECK(row.valid);
+  PLOT_CHECK_NEAR(row.x, 2.0, 1e-12);
+  PLOT_CHECK_NEAR(row.y, 20.0, 1e-9);
+  const f32 sample_col = static_cast<f32>(scene.plot.x_transform(scene.ax).to_screen_x(2.0));
+  const f32 pointer_col = static_cast<f32>(scene.plot.x_transform(scene.ax).to_screen_x(1.8));
+  PLOT_CHECK_NEAR(row.marker.x, sample_col, 1e-3);
+  PLOT_CHECK_NEAR(row.marker.y,
+                  static_cast<f32>(scene.plot.y_transform(scene.ay).to_screen_y(20.0)), 1e-3);
+  // The two columns are far apart at this scale, so the check above is not satisfied by both
+  // being nearly the same number.
+  PLOT_CHECK(std::abs(row.marker.x - pointer_col) > 10.0f);
+  // The vertical line stays on the *pointer*: it is where the hover is, and only the mark moved.
+  PLOT_CHECK_NEAR(scene.plot.layout().crosshair.vertical.x +
+                      0.5f * scene.plot.layout().crosshair.vertical.w,
+                  pointer_col, 1e-3);
+}
+
+/// The other mode, and the reason it exists: `CrosshairValue::Interpolated` answers "what is the
+/// signal *at* this instant" by blending the two bracketing samples, for a caller whose data is a
+/// continuous quantity sampled finely enough that the line between two samples is a model it
+/// believes. `NearestSample` is the default because that line is an artefact of plotting for most
+/// sources -- see `CrosshairValue` -- but the question is about the data, so the library offers
+/// both rather than picking for the caller.
+///
+/// Every number here is one the *other* mode refuses, so the two are pinned apart rather than
+/// merely exercised: 15 at the tie, 11 and 19 either side of it. The marker moves with the value,
+/// which is the mode's second half -- an interpolated y belongs at the pointer's x, on the
+/// polyline, and nowhere else.
+PLOT_TEST(crosshair_interpolated_mode_blends_the_two_samples) {
+  CrosshairScene scene;
+  const u32 trace = scene.plot.add_trace(
+      {&scene.series, plot::Color::hex(0x40C0FF), 1.5f, scene.ax, scene.ay, "sine"});
+  PLOT_CHECK(scene.plot.crosshair_value_mode() == plot::CrosshairValue::NearestSample);
+  scene.plot.set_crosshair_value_mode(plot::CrosshairValue::Interpolated);
+  PLOT_CHECK(scene.plot.crosshair_value_mode() == plot::CrosshairValue::Interpolated);
+
+  f64 y = 0.0;
+  PLOT_CHECK(scene.plot.crosshair_value(trace, 1.5, y));
+  PLOT_CHECK_NEAR(y, 15.0, 1e-9);
+  PLOT_CHECK(scene.plot.crosshair_value(trace, 1.1, y));
+  PLOT_CHECK_NEAR(y, 11.0, 1e-9);
+  PLOT_CHECK(scene.plot.crosshair_value(trace, 1.9, y));
+  PLOT_CHECK_NEAR(y, 19.0, 1e-9);
+  // A query *on* a sample reads that sample back exactly in this mode too: the span collapses to
+  // one index, `t` is 0, and `crosshair_interpolate()`'s form returns `y_begin` unrounded.
+  PLOT_CHECK(scene.plot.crosshair_value(trace, 2.0, y));
+  PLOT_CHECK(y == 20.0);
+  PLOT_CHECK(plot::crosshair_interpolate(10.0, 20.0, 0.0) == 10.0);
+  PLOT_CHECK(plot::crosshair_interpolate(10.0, 20.0, 1.0) == 20.0);
+  PLOT_CHECK_NEAR(plot::crosshair_interpolate(10.0, 20.0, 0.25), 12.5, 1e-12);
+  // Out of the window is still no value: interpolating needs two samples to be between.
+  PLOT_CHECK(!scene.plot.crosshair_value(trace, 4.0, y));
+
+  // The point is the *query's* x here, not a sample's, so the mark sits on the polyline where the
+  // number is -- the opposite placement from `NearestSample`, and the mode's visible half.
+  f64 px = 0.0;
+  f64 py = 0.0;
+  PLOT_CHECK(scene.plot.crosshair_point(trace, 1.8, px, py));
+  PLOT_CHECK_NEAR(px, 1.8, 1e-12);
+  PLOT_CHECK_NEAR(py, 18.0, 1e-9);
+  scene.plot.set_crosshair(1.8, true);
+  scene.plot.layout();
+  const plot::CrosshairRow& row = scene.plot.layout().crosshair.rows[0];
+  PLOT_CHECK_NEAR(row.x, 1.8, 1e-12);
+  PLOT_CHECK(row.value == "18");
+  const f32 pointer_col = static_cast<f32>(scene.plot.x_transform(scene.ax).to_screen_x(1.8));
+  PLOT_CHECK_NEAR(row.marker.x, pointer_col, 1e-3);
+
+  // Switching back is a plain state change, with no second placement to keep in step: the next
+  // layout reports the sample again.
+  scene.plot.set_crosshair_value_mode(plot::CrosshairValue::NearestSample);
+  scene.plot.layout();
+  PLOT_CHECK_NEAR(scene.plot.layout().crosshair.rows[0].x, 2.0, 1e-12);
+  PLOT_CHECK_NEAR(scene.plot.layout().crosshair.rows[0].y, 20.0, 1e-9);
+}
+
+/// The mark has to be **bigger than a trace's own sample dots**, because in the default mode it is
+/// snapped onto one of them: at the same size it reads as one more sample rather than as the one
+/// the panel is quoting. That is a property of the defaults, not of a drawing call, so it is
+/// asserted where it is decided -- a later edit that tunes `marker_px` up past the crosshair's is
+/// the regression, and nothing else in the library would notice.
+PLOT_TEST(the_crosshair_mark_is_bigger_than_a_sample_dot) {
+  const plot::render::Style style;
+  PLOT_CHECK(style.crosshair_marker_px > style.marker_px);
+  // And heavier than the crosshair's own lines, which are chrome and recede on purpose.
+  PLOT_CHECK(style.crosshair_marker_width > style.crosshair_width);
+}
+
 /// Spec §15, the readout's membership: **every visible trace gets a row, whether or not it is
 /// named** -- the readout is about what is under the pointer, so the legend's rule of skipping
 /// unnamed traces does not apply -- while a trace with no series or no samples is not under the
@@ -2343,9 +2471,9 @@ class XyFixture final : public plot::data::Series {
   }
   plot::Range y_bounds() const override { return {0.0, 20.0}; }
   const plot::data::MinMaxPyramid& pyramid() const override { return pyramid_; }
-  void gather(u32 level, u64 begin, u64 end, std::vector<f32>& out,
+  void gather(u32 level, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out,
               plot::data::GatherMode mode = plot::data::GatherMode::Envelope) const override {
-    gather_impl(level, begin, end, out, mode);
+    gather_impl(level, begin, end, x_origin, out, mode);
   }
 
   f64 x_at(u64 i) const override { return x_[static_cast<std::size_t>(i)]; }

tests/test_render_smoke.cpp

@@ -723,11 +723,11 @@ class LyingGridSeries final : public plot::data::Series {
     if (x_at(i0) != x && i0 + 1u < kN) i1 = i0 + 1u;
     return true;
   }
-  void gather(u32, u64 begin, u64 end, std::vector<f32>& out,
+  void gather(u32, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out,
               plot::data::GatherMode) const override {
     ++gather_calls_;
     for (u64 i = begin; i < end; ++i) {
-      out.push_back(static_cast<f32>(x_at(i) - x0()));
+      out.push_back(static_cast<f32>(x_at(i) - x_origin));
       out.push_back(kY);
     }
   }
@@ -1512,9 +1512,10 @@ PLOT_TEST(a_clamped_front_does_not_draw_a_segment_across_the_panel) {
       if (x_at(k) != x && k + 1u < 8u) i1 = k + 1u;
       return true;
     }
-    void gather(u32, u64 begin, u64 end, std::vector<f32>& out, plot::data::GatherMode) const override {
+    void gather(u32, u64 begin, u64 end, f64 x_origin, std::vector<f32>& out,
+                plot::data::GatherMode) const override {
       for (u64 i = begin; i < end; ++i) {
-        out.push_back(static_cast<f32>(x_at(i) - x0()));
+        out.push_back(static_cast<f32>(x_at(i) - x_origin));
         out.push_back(i < 2u ? -1.0f : 1.0f);
       }
     }
@@ -3171,6 +3172,111 @@ PLOT_TEST(legend_does_not_cover_the_plot_when_placed_bottom_left) {
   ctx.destroy();
 }
 
+/// `Plot::set_x_note()`'s caption is *chrome*: it sits in the bottom-left corner of the
+/// axes on the legend's own backplate, and panning or zooming the x range does not move
+/// it by a pixel. That is the whole reason it exists rather than an `Annotation`, whose
+/// anchor is a data coordinate and so crawls under a pan and drifts under a zoom.
+///
+/// The caption's rect is deliberately not readable back -- it reserves no space and
+/// nothing can be picked on it -- so the test measures it the only way a reader can see
+/// it: the pixels a frame *with* the note differs from the same frame without one. The
+/// bounding box of that difference is the caption's footprint, and the assertion is that
+/// two completely different x ranges produce the *same* box.
+PLOT_TEST(the_x_note_is_chrome_that_a_pan_or_a_zoom_does_not_move) {
+  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));
+
+  plot::data::ArraySeries s(sine_samples(), 0.0, 1.0);
+  plot::Plot p;
+  p.set_viewport({0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  const plot::u32 ax = p.add_x_axis({"t", s.x_bounds(), true, plot::AxisPlace::Bottom, false});
+  const plot::u32 ay = p.add_y_axis({"v", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
+  PLOT_CHECK(p.add_trace({&s, plot::Color::hex(0x00E070, 1.0f), 1.5f, ax, ay, "signal"}) == 0u);
+
+  // The footprint of the note at one x range: the bounding box of the pixels that the
+  // caption, and only the caption, is responsible for.
+  const auto footprint = [&](const plot::Range& view, const char* dump) {
+    plot::Rect box{0.0f, 0.0f, 0.0f, 0.0f};
+    p.set_x_range(ax, view);
+    p.set_x_note("t0 = 1.23457e+06 s");
+    r.begin_frame(kW, kH, kFrameRect);
+    r.draw(p);
+    r.end_frame();
+    PLOT_CHECK(!r.last_frame_had_gl_error());
+    dump_if_requested(r, dump);
+    const std::vector<plot::u8> with = read_frame(ctx);
+    p.set_x_note("");
+    r.begin_frame(kW, kH, kFrameRect);
+    r.draw(p);
+    r.end_frame();
+    PLOT_CHECK(!r.last_frame_had_gl_error());
+    const std::vector<plot::u8> without = read_frame(ctx);
+    plot::u32 lo_x = kW, lo_y = kH, hi_x = 0, hi_y = 0, differing = 0;
+    for (plot::u32 y = 0; y < kH; ++y) {
+      for (plot::u32 x = 0; x < kW; ++x) {
+        const plot::usize at = (plot::usize(y) * kW + x) * 4u;
+        if (with[at] == without[at] && with[at + 1u] == without[at + 1u] &&
+            with[at + 2u] == without[at + 2u] && with[at + 3u] == without[at + 3u]) {
+          continue;
+        }
+        ++differing;
+        lo_x = std::min(lo_x, x);
+        lo_y = std::min(lo_y, y);
+        hi_x = std::max(hi_x, x);
+        hi_y = std::max(hi_y, y);
+      }
+    }
+    // A comparison of two frames is only evidence if the caption was in one of them.
+    PLOT_CHECK(differing > 0u);
+    box = plot::Rect{static_cast<plot::f32>(lo_x), static_cast<plot::f32>(lo_y),
+                     static_cast<plot::f32>(hi_x - lo_x + 1u),
+                     static_cast<plot::f32>(hi_y - lo_y + 1u)};
+    return box;
+  };
+
+  const plot::Rect wide = footprint({0.0, 640.0}, "x_note.ppm");
+  // The corner: inside the axes, held off the left and bottom edges by a pad of a few
+  // pixels, and nowhere near the right-hand side or the top.
+  const plot::Rect axes = p.layout().axes;
+  PLOT_CHECK(wide.x > axes.x && wide.x < axes.x + 16.0f);
+  PLOT_CHECK(wide.bottom() <= axes.bottom() && wide.bottom() > axes.bottom() - 16.0f);
+  PLOT_CHECK(wide.right() < axes.x + 0.5f * axes.w);  // a caption, not a banner
+  PLOT_CHECK(wide.h >= 8.0f && wide.h < 0.5f * axes.h);
+  // Real glyph ink on the backplate, so the box is a caption and not an empty panel. The
+  // frame has to be re-drawn *with* the note first: the measurement above left the
+  // note-less frame in the buffer, and `scan_rect_ink` reads the live one.
+  p.set_x_range(ax, {0.0, 640.0});
+  p.set_x_note("t0 = 1.23457e+06 s");
+  r.begin_frame(kW, kH, kFrameRect);
+  r.draw(p);
+  r.end_frame();
+  PLOT_CHECK(!r.last_frame_had_gl_error());
+  PLOT_CHECK(scan_rect_ink(ctx, wide, r.style().axis_text).pixels >= 16u);
+
+  // The same caption under a pan to a different decade and a 64x zoom: an annotation
+  // anchored in data would have left the frame entirely, and the one here has not moved
+  // a pixel. The text is fixed across the three, since this case is about the position.
+  const plot::Rect panned = footprint({320.0, 960.0}, "x_note_panned.ppm");
+  const plot::Rect zoomed = footprint({300.0, 310.0}, "x_note_zoomed.ppm");
+  PLOT_CHECK_NEAR(panned.x, wide.x, 1e-4);
+  PLOT_CHECK_NEAR(panned.y, wide.y, 1e-4);
+  PLOT_CHECK_NEAR(panned.w, wide.w, 1e-4);
+  PLOT_CHECK_NEAR(panned.h, wide.h, 1e-4);
+  PLOT_CHECK_NEAR(zoomed.x, wide.x, 1e-4);
+  PLOT_CHECK_NEAR(zoomed.y, wide.y, 1e-4);
+  PLOT_CHECK_NEAR(zoomed.w, wide.w, 1e-4);
+  PLOT_CHECK_NEAR(zoomed.h, wide.h, 1e-4);
+  std::printf("[x note] footprint %.0f,%.0f %.0fx%.0f in axes %.0f,%.0f %.0fx%.0f; panned "
+              "%.0f,%.0f %.0fx%.0f, zoomed %.0f,%.0f %.0fx%.0f\n",
+              wide.x, wide.y, wide.w, wide.h, axes.x, axes.y, axes.w, axes.h, panned.x, panned.y,
+              panned.w, panned.h, zoomed.x, zoomed.y, zoomed.w, zoomed.h);
+  ctx.destroy();
+}
+
 // --- one physical size at any DPI --------------------------------------------
 
 namespace {

tests/test_series.cpp

@@ -24,7 +24,7 @@ PLOT_TEST(array_series_bounds_and_gather) {
   PLOT_CHECK_NEAR(s.x_bounds().max, 11.5, 1e-9);
   PLOT_CHECK_NEAR(s.y_bounds().min, -2.0, 1e-6);
   std::vector<f32> out;
-  s.gather(0, 0, 4, out);
+  s.gather(0, 0, 4, s.x0(), out);
   PLOT_CHECK(out.size() == 8);             // interleaved x,y
   // x is relative to x0(): sample 0 emits 0, not 10 (the relative-x contract).
   PLOT_CHECK_NEAR(out[0], 0.0, 1e-6);
@@ -104,7 +104,7 @@ PLOT_TEST(stream_series_exact_queries_survive_ring_wrap) {
   PLOT_CHECK_NEAR(s.sumsq(0, n), sumsq, 1e-1);
 
   std::vector<f32> out;
-  s.gather(0, 0, n, out);                               // level 0: raw samples
+  s.gather(0, 0, n, s.x0(), out);                               // level 0: raw samples
   PLOT_CHECK(out.size() == 2 * n);
   // x is relative to x0(): the oldest retained sample emits 0, the newest
   // (n - 1) * dx, never the absolute data coordinates (relative-x contract).
@@ -157,7 +157,7 @@ PLOT_TEST(stream_series_push_and_read_are_race_free) {
       (void)s.sum(0, n);
       (void)s.sumsq(0, n);
       std::vector<f32> out;
-      s.gather(0, 0, n, out);                           // reads the oldest edge
+      s.gather(0, 0, n, s.x0(), out);                           // reads the oldest edge
       if (out.size() != 2 * n) sizes_ok = false;
     }
     ++reads;
@@ -258,7 +258,7 @@ PLOT_TEST(stream_series_sliding_window_queries_match_bruteforce) {
   // wholly at or after window index 0, `last` one past the last wholly inside
   // the window -- so every emitted bucket's samples lie inside the window.
   std::vector<f32> out;
-  s.gather(1, 0, n, out);
+  s.gather(1, 0, n, s.x0(), out);
   const u64 first =
       std::max(base / span + (base % span == 0 ? u64{0} : u64{1}), lbase);   // ceil, then clamp
   const u64 last = std::min((base + n) / span, lbase + lsize);
@@ -374,7 +374,7 @@ PLOT_TEST(gather_at_a_lod_level_reaches_both_ends_of_the_window) {
 
   const auto check_ends = [&](u32 level) {
     std::vector<f32> out;
-    s.gather(level, 0, n, out);
+    s.gather(level, 0, n, s.x0(), out);
     PLOT_CHECK(out.size() >= 8u);
     // Oldest: the first emitted pair is the retained sample at index 0, not the
     // first whole bucket (which starts `lead` samples later). x is relative to
@@ -403,7 +403,7 @@ PLOT_TEST(gather_at_a_lod_level_reaches_both_ends_of_the_window) {
   PLOT_CHECK((ny % 256u) != 0u);   // 5000 = 19 * 256 + 136 at level 2
   plot::data::ArraySeries a(y, 0.0, 1e-6);
   std::vector<f32> out;
-  a.gather(2, 0, ny, out);
+  a.gather(2, 0, ny, a.x0(), out);
   PLOT_CHECK(out.size() >= 8u);
   PLOT_CHECK_NEAR(out[0], 0.0, 1e-9);                       // level 2 starts on the grid
   PLOT_CHECK_NEAR(out[out.size() - 2], f32(f64(ny - 1) * 1e-6), 1e-9);
@@ -469,7 +469,7 @@ PLOT_TEST(gather_envelope_stays_inside_an_unaligned_range) {
 
   const auto check_range = [&](u32 level) {
     std::vector<f32> out;
-    s.gather(level, b, e, out);
+    s.gather(level, b, e, s.x0(), out);
     PLOT_CHECK(out.size() >= 4u);
     for (plot::usize i = 0; i + 1 < out.size(); i += 2) {
       PLOT_CHECK(f64(out[i]) >= f64(b));    // emitted x inside [begin, end)
@@ -499,7 +499,7 @@ PLOT_TEST(gather_envelope_stays_inside_an_unaligned_range) {
   const u64 buckets = last - first;
   PLOT_CHECK(lead > b && last * span < e);   // both edge spans really are partial
   std::vector<f32> out;
-  s.gather(level, b, e, out);
+  s.gather(level, b, e, s.x0(), out);
   PLOT_CHECK(out.size() == 2 * ((lead - b) + 2 * buckets + (e - last * span)));
   plot::usize o = 0;
   for (u64 i = b; i < lead; ++i, o += 2) {   // the partial leading bucket, raw
@@ -544,7 +544,7 @@ PLOT_TEST(stream_gather_envelope_stays_inside_an_unaligned_range) {
   const plot::Range exact = s.reduce(b, e);
   PLOT_CHECK(exact.max < 1.0 && exact.min > -1.0);
   std::vector<f32> out;
-  s.gather(1, b, e, out);
+  s.gather(1, b, e, s.x0(), out);
   PLOT_CHECK(out.size() >= 4u);
   for (plot::usize i = 0; i + 1 < out.size(); i += 2) {
     PLOT_CHECK(f64(out[i]) >= f64(b));              // relative x inside [begin, end)
@@ -614,11 +614,11 @@ PLOT_TEST(gather_level_beyond_the_pyramid_clamps) {
   for (u64 i = 0; i < y.size(); ++i) y[i] = f32(i % 13) - 6.0f;
   plot::data::ArraySeries s(std::move(y), 0.0, 1.0);
   std::vector<f32> deepest;
-  s.gather(plot::data::kMaxPyramidLevel, 0, s.size(), deepest);
+  s.gather(plot::data::kMaxPyramidLevel, 0, s.size(), s.x0(), deepest);
   PLOT_CHECK(!deepest.empty());
   for (const u32 level : {u32{58}, u32{200}, u32{1000}, ~u32{0}}) {
     std::vector<f32> out;
-    s.gather(level, 0, s.size(), out);
+    s.gather(level, 0, s.size(), s.x0(), out);
     PLOT_CHECK(out == deepest);   // clamped to the same deepest geometry
   }
   // The cap is where the span shift still fits: level 57 spans exactly 2^63.
@@ -657,9 +657,9 @@ PLOT_TEST(gather_modes_envelope_nearest_mean) {
   PLOT_CHECK(buckets == 16);
 
   std::vector<f32> env, near, mean;
-  s.gather(level, 0, kN, env, plot::data::GatherMode::Envelope);
+  s.gather(level, 0, kN, s.x0(), env, plot::data::GatherMode::Envelope);
-  s.gather(level, 0, kN, near, plot::data::GatherMode::Nearest);
+  s.gather(level, 0, kN, s.x0(), near, plot::data::GatherMode::Nearest);
-  s.gather(level, 0, kN, mean, plot::data::GatherMode::Mean);
+  s.gather(level, 0, kN, s.x0(), mean, plot::data::GatherMode::Mean);
 
   // Envelope: two points per bucket, min at the bucket's first sample, max at
   // its last (this sawtooth's per-bucket min is its first sample's value and
@@ -692,7 +692,7 @@ PLOT_TEST(gather_modes_envelope_nearest_mean) {
 
   // Level 0 and sub-bucket ranges are the raw samples for every mode.
   std::vector<f32> raw;
-  s.gather(0, 5, 300, raw, plot::data::GatherMode::Mean);
+  s.gather(0, 5, 300, s.x0(), raw, plot::data::GatherMode::Mean);
   PLOT_CHECK(raw.size() == 2 * (300 - 5));
   PLOT_CHECK_NEAR(raw[0], 5.0, 1e-9);
 }
@@ -712,17 +712,17 @@ PLOT_TEST(gather_skips_all_nan_buckets) {
 
   // first = 0, last = 256/128 = 2: buckets 0 and 1, no lead/trail raw spans.
   std::vector<f32> env, mean, near;
-  s.gather(1, 0, 256, env, plot::data::GatherMode::Envelope);
+  s.gather(1, 0, 256, s.x0(), env, plot::data::GatherMode::Envelope);
   PLOT_CHECK(env.size() == 4);              // bucket 0's two pairs only
   PLOT_CHECK_NEAR(env[0], 0.0, 1e-9);
   PLOT_CHECK_NEAR(env[1], 1.0, 1e-9);
   PLOT_CHECK_NEAR(env[2], 127.0, 1e-9);
   PLOT_CHECK_NEAR(env[3], 1.0, 1e-9);
-  s.gather(1, 0, 256, mean, plot::data::GatherMode::Mean);
+  s.gather(1, 0, 256, s.x0(), mean, plot::data::GatherMode::Mean);
   PLOT_CHECK(mean.size() == 2);             // bucket 1 has no finite mean: skipped
   PLOT_CHECK_NEAR(mean[0], 64.0, 1e-9);
   PLOT_CHECK_NEAR(mean[1], 1.0, 1e-9);
-  s.gather(1, 0, 256, near, plot::data::GatherMode::Nearest);
+  s.gather(1, 0, 256, s.x0(), near, plot::data::GatherMode::Nearest);
   PLOT_CHECK(near.size() == 4);             // every bucket still has a centre sample
   PLOT_CHECK_NEAR(near[0], 64.0, 1e-9);
   PLOT_CHECK_NEAR(near[1], 1.0, 1e-9);
@@ -739,7 +739,7 @@ PLOT_TEST(stream_series_freeze_copies_the_last_acquire_window) {
   const u64 n = s.size();
   const plot::Range bounds = s.y_bounds();
   std::vector<f32> gathered;
-  s.gather(1, 0, n, gathered);
+  s.gather(1, 0, n, s.x0(), gathered);
   const plot::Range reduced = s.reduce(0, n);
 
   // Freeze now, while the ring still holds exactly the published window.
@@ -754,7 +754,7 @@ PLOT_TEST(stream_series_freeze_copies_the_last_acquire_window) {
   PLOT_CHECK_NEAR(fr.max, reduced.max, 1e-6);
   PLOT_CHECK_NEAR(frozen.sum(0, n), s.sum(0, n), 1e-3);
   std::vector<f32> frozen_gathered;
-  frozen.gather(1, 0, n, frozen_gathered);
+  frozen.gather(1, 0, n, frozen.x0(), frozen_gathered);
   PLOT_CHECK(frozen_gathered == gathered);   // same geometry, byte for byte
 
   // The producer now laps the ring (700 more pushes overwrite the oldest
@@ -769,6 +769,46 @@ PLOT_TEST(stream_series_freeze_copies_the_last_acquire_window) {
   PLOT_CHECK(frozen.reduce(0, n).max == reduced.max);
 }
 
+// --- The gather origin: f32 precision against the view, not the window -------
+
+PLOT_TEST(a_gather_origin_at_the_view_keeps_a_deep_zoom_on_the_sample_it_quotes) {
+  // 20 s of retained data at 1 kS/s examined through a 10 us window -- the shape of
+  // a scope zoomed in on a long ring. gather() stages x as f32, so what the origin
+  // decides is the span that f32 has to resolve: the whole retained window (which
+  // is what `x0()`, the old implicit origin, names) or the part of it on screen. A
+  // readout quotes `x_at()` in exact f64, so the difference between the two is the
+  // distance between where a sample is *drawn* and the x reported for it.
+  constexpr u64 kN = 20001;
+  std::vector<f32> y(static_cast<plot::usize>(kN));
+  for (u64 i = 0; i < kN; ++i) y[static_cast<plot::usize>(i)] = f32(i % 71);
+  const plot::data::ArraySeries s(std::move(y), /*x0=*/0.0, /*dx=*/1e-3);
+  PLOT_CHECK_NEAR(s.x_bounds().max, 20.0, 1e-9);
+
+  const f64 centre = s.x_at(kN - 2u);                      // 19.999 s, near the far end
+  const plot::Range view{centre - 5e-6, centre + 5e-6};    // 10 us across
+  const f64 px_per_x = 700.0 / (view.max - view.min);      // a typical axis width
+
+  const auto worst_error_px = [&](f64 origin) {
+    std::vector<f32> out;
+    s.gather(0, kN - 3u, kN, origin, out);
+    PLOT_CHECK(out.size() == 6u);
+    f64 worst = 0.0;
+    for (u64 k = 0; k < 3u; ++k) {
+      // Exactly the renderer's reconstruction: the origin added back in f64.
+      const f64 drawn = origin + f64(out[static_cast<plot::usize>(2u * k)]);
+      worst = std::max(worst, std::abs(drawn - s.x_at(kN - 3u + k)) * px_per_x);
+    }
+    return worst;
+  };
+
+  // The visible range's minimum: the f32 resolves 2^-24 of what is on screen, so
+  // the error is a fixed fraction of a pixel at every zoom.
+  PLOT_CHECK(worst_error_px(view.min) < 0.05);
+  // The start of the retained window -- what every caller used to get -- resolves
+  // 2^-24 of 20 s instead, which at this zoom is most of the axis.
+  PLOT_CHECK(worst_error_px(s.x0()) > 10.0);
+}
+
 // --- Incremental append: ArraySeries::push() ----------------------------------
 
 PLOT_TEST(array_series_push_matches_commit) {

tests/test_ticks.cpp

@@ -263,6 +263,18 @@ PLOT_TEST(format_time_never_lets_a_field_reach_its_radix) {
   plot::format_time(86399.5, b, sizeof b);  PLOT_CHECK(std::string(b) == "1d");
   plot::format_time(86400.0, b, sizeof b);  PLOT_CHECK(std::string(b) == "1d");
 
+  // A mantissa may not reach its own radix either: three significant figures of a value a
+  // hair under a rung's top round up to 1000, and `%.3g` writes that `1e+03`. A one-second
+  // tick computed as 0.9999999 s read `1e+03ms` on a live axis, which is where this was seen.
+  plot::format_time(0.9999999, b, sizeof b);  PLOT_CHECK(std::string(b) == "1s");
+  plot::format_time(0.9995, b, sizeof b);     PLOT_CHECK(std::string(b) == "1s");
+  plot::format_time(0.9994, b, sizeof b);     PLOT_CHECK(std::string(b) == "999ms");
+  plot::format_time(0.9999999e-3, b, sizeof b); PLOT_CHECK(std::string(b) == "1ms");
+  plot::format_time(0.9994e-3, b, sizeof b);  PLOT_CHECK(std::string(b) == "999\u00b5s");
+  plot::format_time(0.9999999e-6, b, sizeof b); PLOT_CHECK(std::string(b) == "1\u00b5s");
+  plot::format_time(0.9999999e-9, b, sizeof b); PLOT_CHECK(std::string(b) == "1ns");
+  plot::format_time(-0.9999999, b, sizeof b); PLOT_CHECK(std::string(b) == "-1s");
+
   // Sweep the boundary neighbourhoods and check every emitted field.
   const double values[] = {0.4, 0.9995, 1.0, 59.4, 59.4999, 59.5, 59.9, 60.0, 90.0, 119.5, 3599.4,
                            3599.5, 3599.9, 86399.4, 86399.5, 86399.9, 86400.0, 172800.0, -119.5,

tests/test_xy_series.cpp

@@ -52,7 +52,8 @@ PLOT_TEST(xy_series_reports_its_own_grid) {
   PLOT_CHECK_NEAR(s.y_bounds().max, 48.0, 1e-6);
 
   // Epoch-style offset: the same shape shifted by 1.7e9 keeps exact bounds, and
-  // gather() emits x relative to x0() -- small numbers, not quantised epochs.
+  // gather() emits x relative to the origin the caller names -- x0() here, so
+  // small numbers, not quantised epochs.
   std::vector<f64> epoch = spread_x(500);
   const std::vector<f64> es = epoch;   // un-moved copy for the expectations
   for (auto& v : epoch) v += 1.7e9;
@@ -60,7 +61,7 @@ PLOT_TEST(xy_series_reports_its_own_grid) {
   PLOT_CHECK_NEAR(e.x_bounds().min, 1.7e9, 1e-3);
   PLOT_CHECK_NEAR(e.x_bounds().max, 1.7e9 + (xs.back() - xs.front()), 1e-3);
   std::vector<f32> out;
-  e.gather(0, 0, 500, out);
+  e.gather(0, 0, 500, e.x0(), out);
   PLOT_CHECK(out.size() == 1000);
   // x is relative to x0(): x_at(i) - x0(), the array's own offsets -- small
   // values even though the absolute epoch is 1.7e9. The expectations come from
@@ -186,7 +187,7 @@ PLOT_TEST(xy_series_gather_envelope_matches_exact_reduces) {
   const u64 span = u64{kBucket} << level;
   const u64 b = 500 + 33, e = 3500 - 57;   // unaligned on the bucket grid
   std::vector<f32> out;
-  s.gather(level, b, e, out, plot::data::GatherMode::Envelope);
+  s.gather(level, b, e, s.x0(), out, plot::data::GatherMode::Envelope);
   PLOT_CHECK(out.size() >= 8u);
   // Emitted x stays inside [x_at(begin), x_at(end)) and the geometry reaches
   // both the range's extremes (the lead/trail raw spans carry them).