feat(plot): the readout marker snaps to the sample it quotes, and the value mode is a choice

The mark was placed at the *pointer's* x with the quoted sample's y, so with the
nearest-sample readout it sat off the datum horizontally -- and further off the deeper
the zoom. `CrosshairRow` now carries the data `x` the value belongs to and the marker is
placed from it, so in `NearestSample` the mark lands on the sample the panel is quoting.
`Style::crosshair_marker_px` (9) and `crosshair_marker_width` (1.5) replace the
renderer's hard-coded 3 px arm and 1 px stroke: a snapped mark lands among a trace's own
sample dots (`marker_px`, 3) and at that size would read as one more sample rather than
as the answer to the question the hover asks.

`CrosshairValue` makes the previous behaviour reachable rather than deleted. The two
modes answer different questions -- `NearestSample` "which datum is under the pointer",
`Interpolated` "what is the signal at this instant" -- and only the caller knows which it
is asking, so the plot offers both with the sample as the default.
`crosshair_interpolate()` is restored for it.

`Plot::crosshair_point()` is the lookup with both coordinates, public because a host that
draws instruments of its own needs the same snap and a second rule for it would drift
from this one; `crosshair_value()` delegates to it.

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,
-  x)` returns the two samples `x` lies between (`begin`, `end`, `t`), and the readout takes
-  whichever of the two is nearer: `t <= 0.5` the earlier, otherwise the later. It answers
-  "which datum is under the pointer", not "what is the signal *at* this instant" — a
-  linearly interpolated y is a number the series does not contain, and on a source whose
-  samples are already averaged or decimated at ingest it is a claim about a signal nobody
-  measured. The lookup is O(1) and allocation-free, and it reads level 0, so the value is a
-  stored sample and never an LOD bucket's aggregate; `Plot::crosshair_value(trace, x,
-  out_y)` exposes the same number to a host, and `CrosshairRow::marker` is placed from that
-  same y, so 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.
+- **The crosshair readout is a single sample — the nearest one — by default.**
+  `crosshair_span(series, x)` returns the two samples `x` lies between (`begin`, `end`, `t`),
+  and `CrosshairValue::NearestSample` takes whichever of the two is nearer: `t <= 0.5` the
+  earlier, otherwise the later. It answers "which datum is under the pointer", not "what is
+  the signal *at* this instant" — a linearly interpolated y is a number the series does not
+  contain, and on a source whose samples are already averaged or decimated at ingest it is a
+  claim about a signal nobody measured. That is why it is the default; a host whose x is a
+  quantity it is willing to interpolate states `CrosshairValue::Interpolated` and gets
+  `crosshair_interpolate(y_begin, y_end, t)` instead, at the crosshair's own x. The lookup is
+  O(1) and allocation-free either way, and it reads level 0, so the value is a stored sample
+  (or a blend of two) and never an LOD bucket's aggregate. `Plot::crosshair_point(trace, x,
+  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
-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
-fabricated value.
+### What a readout is: the two value modes
+
+`Crosshair::value_mode` (`set_crosshair_value_mode()`) answers two different questions, and
+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/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
-/// whichever of the two samples adjacent to `x` is **nearer** to it -- not a value
-/// interpolated between them, and not an interval statistic. See `crosshair_span()` for the
-/// rule stated in full, and for how it differs from
-/// `Series::reduce()`/`sum()`/`sumsq()`.
+/// **The readout's values are samples by default** -- the y a row reports is the nearer of
+/// the two samples adjacent to `x`, and its marker sits on that sample. `value_mode` is what
+/// chooses otherwise; see `CrosshairValue` for the two questions it picks between, and
+/// `crosshair_span()` for how both differ from `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
-/// builds from this span is the y of whichever end `t` is nearer to (`Plot::crosshair_value()`
-/// takes `t <= 0.5` as `begin`), so a readout quotes a sample the series actually holds. It
-/// used to be a linear interpolation between the two, which reads as "the signal at this
-/// instant" -- a number that is *between* the data and therefore in no row of it. On a
-/// decimated or averaged source that is a value nothing measured, and at a zoom past the
-/// sample spacing it is visibly not the datum the pointer is over.
+/// **`t` is a position, not yet an answer.** It says where the query sits between the two
+/// samples and nothing more; what a caller makes of it is `CrosshairValue`'s to decide. In
+/// `NearestSample` -- the default -- it *selects*, `t <= 0.5` taking `begin`, so the readout
+/// quotes a sample the series actually holds. In `Interpolated` it *blends*, through
+/// `crosshair_interpolate()`. A plot cannot tell which is right, because the question is
+/// about the data and not about the drawing; `CrosshairValue` is where that is argued.
 ///
-/// So `t` is a *selector*, and this remains a different convention from
-/// `Series::reduce()`, `sum()` and `sumsq()`, which are **exact over a sample range**: those
-/// answer "what are the samples' extremes/total over this interval", this answers "which
-/// sample is under the pointer, and what is its value". Both now quote the data; what differs
-/// is whether the answer is one sample or a statistic of many.
+/// Either way this is a different convention from `Series::reduce()`, `sum()` and `sumsq()`,
+/// which are **exact over a sample range**: those answer "what are the samples'
+/// extremes/total over this interval", a crosshair answers "what is the signal at this one
+/// x". What differs is a statistic of many samples against a reading at one place.
 ///
 /// `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
-  /// factor -- see the struct comment.
+  /// The query's position between the two samples, in `[0, 1]`. Whether it selects one of
+  /// 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
-  /// meaningful when `valid`.
+  /// The data x the row's value belongs to, on the trace's own x axis: in
+  /// `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
-  /// 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
-  /// disagreeing. Meaningless when `!valid`.
+  /// scale it is drawn against.
+  ///
+  /// It marks **the point the row reports**, which is the whole job: in
+  /// `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/plot.hpp

@@ -688,23 +688,31 @@ 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
-  /// `x`, and the nearer of the two samples is the answer (`t <= 0.5` takes the earlier) --
-  /// the same lookup the readout's rows are built from, exposed 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.
+  /// The value this plot's readout would report for `trace` at a data `x`: the nearer
+  /// bracketing sample's y, or the blend of the two, according to
+  /// `crosshair_value_mode()` -- the same lookup the readout's rows are built from, exposed
+  /// for a host that wants the number for itself (a status line, a tooltip of its own).
   ///
   /// 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
-  /// `crosshair_span()` for why an out-of-window x is not a value.
+  /// documents, plus the trace itself. Never a clamped answer: see `crosshair_span()` for why
+  /// 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 +720,17 @@ 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

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/plot/plot.cpp

@@ -1410,6 +1410,13 @@ 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 +1446,11 @@ 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;
@@ -1455,12 +1467,23 @@ bool Plot::crosshair_value(u32 trace, f64 x, f64& out_y) const {
   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
-  // number the series does not contain is a claim about a signal between its samples, which
-  // is the caller's to make and not a plot's to put in a panel. `span.t` is measured on the
-  // samples' own x, 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.
-  out_y = span.t <= 0.5 ? y_begin : y_end;
+  if (crosshair_.value_mode == CrosshairValue::Interpolated) {
+    // The signal the samples imply *at* this x, so the point is the query's own x: that is
+    // where an interpolated number lives, and the marker on the polyline there is honest
+    // about it.
+    out_x = x;
+    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
+  // `x0()` 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 +1552,23 @@ 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
-    // where it does not. The row is listed either way, so the absence has to be *said* -- a blank
-    // value cell reads as a value that has not arrived -- and the dash is measured into the panel's
-    // width like any other value, so the row's boxes are placed by one rule.
-    if (crosshair_value(i, crosshair_.x, row.y)) {
+    // The value where the trace has one at this x, and the em dash where it does not. The row is
+    // listed either way, so the absence has to be *said* -- a blank value cell reads as a value
+    // that has not arrived -- and the dash is measured into the panel's width like any other
+    // value, so the row's boxes are placed by one rule.
+    //
+    // 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 +1577,7 @@ 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/render/renderer.cpp

@@ -1522,19 +1522,28 @@ 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
-  // legend's swatch, so a trace drawn at 15% still has a marker that can be seen. The arm
-  // length is a logical size like every other marker's, and the position is the layout's: a
-  // marker for a sample far outside the visible y range is staged where the trace is and the
-  // axes clip keeps it inside, rather than being skipped by a range test of this layer's own.
-  const f32 arm = style_.device(3.0f);
-  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));
+  // A cross at each trace's readout point, in the trace's own colour -- opaque, like a legend's
+  // swatch, so a trace drawn at 15% still has a mark that can be seen. The position is the
+  // layout's, and in the default `CrosshairValue::NearestSample` that is the *quoted sample*
+  // rather than the pointer's column: a mark for a sample outside the visible range is staged
+  // where that sample is and the axes clip keeps it inside, rather than being skipped by a range
+  // 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, mark_stroke);
+    }
   }
 }
 

tests/test_plot.cpp

@@ -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