feat(plot): the figure crosshair marks the hovered row, and the stroke is thin, dashed and configurable

adapters/glfw/glfw_host.cpp

@@ -494,12 +494,15 @@ void Host::sync_figure_crosshair(const std::vector<Rect>& rows) noexcept {
     if (hovered >= row_count) return;
   }
 
-  // Applied to the whole figure: one x, every row. A row whose controller has *no* crosshair (the
-  // pointer left its axes, which is the only thing that clears one) clears the figure's, so the
-  // rows cannot be left showing a hover the pointer has abandoned.
+  // Applied to the whole figure: one x, every row; and the pointer row's **own** y, only to that
+  // row -- so the row under the pointer gets the one horizontal line, and no row ever shows
+  // another row's y (spec §17). The y is the row's own data coordinate, placed by its controller
+  // from the pointer's pixel row, and it is never spread to a sibling. A row whose controller has
+  // *no* crosshair (the pointer left its axes, which is the only thing that clears one) clears the
+  // figure's, so the rows cannot be left showing a hover the pointer has abandoned.
   const Plot& row = figure_->plot(hovered);
   if (row.crosshair().active) {
-    figure_->set_crosshair(row.crosshair().x, true);
+    figure_->set_crosshair(row.crosshair().x, hovered, row.crosshair().y, true);
     spread_x_ = row.crosshair().x;
     spread_active_ = true;
   } else {

include/plot/plot/crosshair.hpp

@@ -9,6 +9,22 @@
 
 namespace plot {
 
+/// A per-`Crosshair` override of the three style knobs spec §17 adds, in **logical**
+/// pixels, so one plot's crosshair can differ -- be thicker, or fatter-dashed -- without a
+/// caller touching the global `render::Style` the rest of the frame draws with.
+///
+/// A negative value means "not overridden": the field falls back to the matching
+/// `Style::crosshair_*` metric, so a default-constructed `CrosshairStyle` (every field
+/// `-1`) is exactly the global style and the common case costs nothing. `width`, `dash_on`
+/// and `dash_off` are independent, so a caller may override only the dash, or only the
+/// width, and leave the rest to the style.
+struct CrosshairStyle {
+  f32 width = -1.0f;     ///< logical stroke width; `< 0` takes `Style::crosshair_width`.
+  f32 dash_on = -1.0f;   ///< logical ink length; `< 0` takes `Style::crosshair_dash_on`.
+  f32 dash_off = -1.0f;  ///< logical gap; `< 0` takes `Style::crosshair_dash_off`, and `0`
+                         ///< (once resolved) means solid.
+};
+
 /// The crosshair's state: where it is, which of its two lines it draws, and whether the
 /// per-signal readout is wanted.
 ///
@@ -46,6 +62,10 @@ 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;
+  /// 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()`.
+  CrosshairStyle style{};
 };
 
 /// The two samples of a uniform-grid series that a data `x` lies between: what the
@@ -177,6 +197,16 @@ struct CrosshairLayout {
   /// The horizontal line's stroke box, the same instrument turned a quarter turn. Zero-sized
   /// when the horizontal line is not drawn.
   Rect horizontal{};
+  /// The stroke width the two lines are drawn at, in **device** pixels: the crosshair's own
+  /// `CrosshairStyle::width` when it overrides it, else `Style::crosshair_width`, both through
+  /// `Style::device()` -- resolved here once, so the stroke boxes above and the renderer's
+  /// strokes are one number rather than two that must agree.
+  f32 stroke = 0.0f;
+  /// The dash pattern in **device** pixels: `x` the ink length, `y` the gap. `y <= 0` means
+  /// solid -- the state whenever no dash is asked for, which is what keeps the shared line
+  /// pipeline drawing every other stroke in the library unchanged. The dash is a fragment-shader
+  /// effect on `v_along`, so this is the only CPU-side change it makes (spec §17).
+  Vec2 dash{};
   /// The readout panel, inside `axes`. Zero-sized when there is nothing to read out (no
   /// visible trace, nothing to measure text with, no room for even one row), when
   /// `Crosshair::show_readout` is off, or when the panel could not be placed inside the

include/plot/plot/figure.hpp

@@ -161,10 +161,33 @@ class Figure {
   /// different things in each row. A row whose series has no sample at `x` lists the trace
   /// with no value rather than inventing one.
   ///
+  /// A host that *does* know which row the pointer is in -- every adapter does, from its own
+  /// pointer position -- uses the four-argument overload below instead, which adds the pointer
+  /// row's own horizontal line; this overload is for a caller that has only a coordinate.
+  ///
   /// A row is placed with `Plot::set_crosshair(x, active)`, which refuses a coordinate that
   /// is not a number; such a call therefore leaves every row's crosshair as it was.
   void set_crosshair(f64 x, bool active);
 
+  /// Places the crosshair with the pointer's **row** and that row's local y (spec §17): the
+  /// shared x goes to every row -- vertical lines everywhere, as the plain overload does -- and
+  /// the horizontal line is drawn **only in `hovered_row`**, where its y is a value of *that*
+  /// row's own scale. It is the fix for the reported defect -- a figure showing no horizontal
+  /// line at all -- without inventing a figure-wide y, which would mean a different value in
+  /// every row: the line appears where the pointer is, and no row ever shows another row's y.
+  ///
+  /// `y_in_row` is a data coordinate on `hovered_row`'s y axis 0, exactly as `Crosshair::y`'s
+  /// two-coordinate placement is; a host reads it from the row's own y transform at the
+  /// pointer's pixel row. `hovered_row` outside `[0, row_count())` is treated as "no row under
+  /// the pointer": every row gets the vertical line and none gets a horizontal one, the same
+  /// state the single-coordinate overload produces.
+  ///
+  /// Refused, changing nothing, when `x` is not a number -- matching `set_crosshair(x, active)`
+  /// and `Plot::set_crosshair()`'s own rule. A non-finite `y_in_row` leaves `hovered_row` with
+  /// the vertical line only (the y is dropped, the hover is not), because a row with no valid y
+  /// is still a row the pointer is in.
+  void set_crosshair(f64 x, u32 hovered_row, f64 y_in_row, bool active);
+
   /// Clears the crosshair on every row. A row that never had one is unaffected.
   void clear_crosshair();
 

include/plot/plot/plot.hpp

@@ -591,6 +591,17 @@ class Plot {
   /// **not** part of `set_crosshair()`, so placing a hover cannot silently change it.
   void set_crosshair_readout(bool on) noexcept { crosshair_.show_readout = on; }
 
+  /// Overrides this plot's crosshair stroke and dash, in logical pixels, without touching the
+  /// global style every other plot draws with (spec §17): each field left negative in `s`
+  /// defers to the matching `Style::crosshair_*` metric, so `CrosshairStyle{}` (the default)
+  /// is exactly the global style. Set it once, like a colour -- it is not part of
+  /// `set_crosshair()`, so placing a hover cannot silently change how the stroke looks. A
+  /// `dash_off` that resolves to `0` draws a solid line.
+  void set_crosshair_style(const CrosshairStyle& s) noexcept { crosshair_.style = s; }
+  /// The crosshair's style override, as set -- read back rather than the *resolved* stroke and
+  /// dash, which live on `CrosshairLayout` because they depend on `ui_scale`.
+  const CrosshairStyle& crosshair_style() const noexcept { return crosshair_.style; }
+
   /// 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.

include/plot/render/program.hpp

@@ -27,6 +27,11 @@ class LineProgram {
   void set_viewport(Vec2 px) const noexcept;
   /// `u_width`: stroke width in pixels.
   void set_width(f32 px) const noexcept;
+  /// `u_dash`: the dash pattern in pixels -- `x` the ink length, `y` the gap, and a
+  /// non-positive `y` (the default a caller passes) means **solid**. It is a fragment-shader
+  /// discard on the interpolated `v_along`, so it costs no geometry, and it is the one uniform
+  /// that distinguishes the crosshair's stroke from every other line in the library.
+  void set_dash(Vec2 on_off) const noexcept;
   /// `u_color`: stroke colour, alpha included.
   void set_color(Color color) const noexcept;
 

include/plot/render/renderer.hpp

@@ -365,7 +365,12 @@ class Renderer {
   /// Uploads the staged line batch, maps it with (`scale`, `offset`), rasterises
   /// it in one colour and empties the batch. A no-op for an empty batch, so the
   /// caller can flush unconditionally.
-  void flush_lines(const Mapping& map, Color color, f32 width) noexcept;
+  ///
+  /// `dash` is the pattern in pixels for the fragment shader's discard: `x` the ink
+  /// length, `y` the gap, and a non-positive `y` -- the default every caller but the
+  /// crosshair passes -- means **solid**. It is set on every flush, so one draw's dash
+  /// can never leak into the next.
+  void flush_lines(const Mapping& map, Color color, f32 width, Vec2 dash = Vec2{}) noexcept;
   /// Draws every staged glyph in `style().axis_text` and empties the batch. Text
   /// is texture-bound, so this runs once per frame, after all line geometry.
   void flush_text() noexcept;

include/plot/render/style.hpp

@@ -54,6 +54,21 @@ struct Style {
   /// already distinct, so no dashes.
   Color crosshair = Color::hex(0xE6EBF2);
 
+  /// The crosshair's stroke and dash, in logical pixels (spec §17). The crosshair is a
+  /// *passive* hover instrument drawn over the data, so its default **recedes**: a
+  /// thinner stroke than a trace's (`Trace::width`'s 1.5) so it reads as chrome rather
+  /// than as data, and a dash so it is distinguishable from the grid and the traces at
+  /// a glance.
+  ///
+  /// `crosshair_dash_on` is the ink length and `crosshair_dash_off` the gap; **`off == 0`
+  /// means solid**, which is the value every *other* line in the library is drawn at, so
+  /// the one pipeline serves them unchanged (the dash is a fragment-shader discard on
+  /// `v_along`, so no geometry is added). A `Crosshair` may override all three for one
+  /// plot without touching this global style.
+  f32 crosshair_width = 0.75f;
+  f32 crosshair_dash_on = 4.0f;
+  f32 crosshair_dash_off = 4.0f;
+
   // --- the scale ----------------------------------------------------------
 
   /// Device pixels per logical unit. 1.0 is the 1x display every default here was

src/plot/figure.cpp

@@ -449,6 +449,22 @@ void Figure::set_crosshair(f64 x, bool active) {
   for (Plot& row : plots_) row.set_crosshair(x, active);
 }
 
+void Figure::set_crosshair(f64 x, u32 hovered_row, f64 y_in_row, bool active) {
+  // The shared x first, on every row -- the same state the single-coordinate overload leaves,
+  // so the vertical lines and every row's readout are placed by one rule whether or not the
+  // pointer's row is known. A bad x is refused by the row call, which changes nothing, so the
+  // y below must not run either or the hover would be half-placed.
+  if (active && !std::isfinite(x)) return;
+  for (Plot& row : plots_) row.set_crosshair(x, active);
+  // Then the pointer row's own y: the *only* row that gets a horizontal line, because the y is
+  // a value on that row's scale and no other row's. A row that does not exist (the pointer is in
+  // the gap between rows) leaves every row vertical-only, which is the honest answer -- there is
+  // no row under the pointer to mark -- and a non-finite y leaves the row vertical-only rather
+  // than clearing a hover the pointer still has.
+  if (!active || hovered_row >= plots_.size()) return;
+  plots_[hovered_row].set_crosshair(x, y_in_row, true);
+}
+
 void Figure::clear_crosshair() {
   for (Plot& row : plots_) row.clear_crosshair();
 }

src/plot/plot.cpp

@@ -1265,6 +1265,8 @@ void Plot::build_crosshair() {
   crosshair.active = crosshair_.active;
   crosshair.vertical = Rect{};
   crosshair.horizontal = Rect{};
+  crosshair.stroke = 0.0f;
+  crosshair.dash = Vec2{};
   crosshair.panel = Rect{};
   if (!crosshair_.active) {
     crosshair.rows.clear();
@@ -1286,7 +1288,18 @@ void Plot::build_crosshair() {
   // rule: the box is where the line may be drawn, and Task 1's clip never has to trim one
   // that already fits. The lines are placed through the primary axis pair, like the grid and
   // a region's fill -- one crosshair is one column and one row of the data area.
-  const f32 stroke = style_.device(style_.line_width_px);
+  //
+  // The stroke and the dash are resolved *here*, once, from the crosshair's own override where
+  // it has one and the global style otherwise, and converted to device pixels through
+  // `device()`. The boxes below and the renderer's strokes are then one set of numbers rather
+  // than two that must be kept in step (spec §17): `stroke` is the device width, `dash` the
+  // device (on, off) pair with `off <= 0` meaning solid.
+  const CrosshairStyle& cs = crosshair_.style;
+  const f32 stroke = style_.device(cs.width >= 0.0f ? cs.width : style_.crosshair_width);
+  crosshair.stroke = stroke;
+  crosshair.dash =
+      Vec2{style_.device(cs.dash_on >= 0.0f ? cs.dash_on : style_.crosshair_dash_on),
+           style_.device(cs.dash_off >= 0.0f ? cs.dash_off : style_.crosshair_dash_off)};
   const f32 half = 0.5f * stroke;
   if (crosshair_.vertical) {
     const f32 at =

src/render/program.cpp

@@ -23,6 +23,10 @@ void LineProgram::set_viewport(Vec2 px) const noexcept { program_.set_uniform("u
 
 void LineProgram::set_width(f32 px) const noexcept { program_.set_uniform("u_width", px); }
 
+void LineProgram::set_dash(Vec2 on_off) const noexcept {
+  program_.set_uniform("u_dash", on_off);
+}
+
 void LineProgram::set_color(Color color) const noexcept { program_.set_uniform("u_color", color); }
 
 bool TextProgram::create(Status* st) noexcept {

src/render/renderer.cpp

@@ -568,7 +568,7 @@ void Renderer::upload_text_vertices(std::span<const f32> interleaved) noexcept {
   text_vao_.upload(interleaved.data(), static_cast<u32>(interleaved.size() / 6u));
 }
 
-void Renderer::flush_lines(const Mapping& map, Color color, f32 width) noexcept {
+void Renderer::flush_lines(const Mapping& map, Color color, f32 width, Vec2 dash) noexcept {
   const u32 segments = lines_.segment_count();
   if (segments != 0 && ready_) {
     upload_vertices(lines_.data());
@@ -577,6 +577,9 @@ void Renderer::flush_lines(const Mapping& map, Color color, f32 width) noexcept
     line_program_.set_offset(map.offset);
     line_program_.set_viewport({viewport_.w, viewport_.h});
     line_program_.set_width(width);
+    // Set on *every* flush, solid included, so the crosshair's dash cannot leak into the
+    // grid, a trace or a cursor drawn after it (see draw_crosshair_lines()).
+    line_program_.set_dash(dash);
     line_program_.set_color(color);
     line_vao_.bind();
     // One triangle strip per instance, four corners from gl_VertexID.
@@ -1192,25 +1195,31 @@ void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout)
   const CrosshairLayout& crosshair = layout.crosshair;
   if (crosshair.empty()) return;
 
-  const f32 stroke = style_.device(style_.line_width_px);
+  // The stroke and dash the layout already resolved from the crosshair's own override and the
+  // global style, in device pixels -- one source, so the boxes the layout held inside `axes` and
+  // the strokes drawn here cannot disagree (spec §17).
+  const f32 stroke = crosshair.stroke;
   // The two lines. Each is staged as one data-space segment at the *centre* of the stroke box
   // the layout placed, spanning that box's own extent, so the pixels this draws are the box
   // rather than a second placement of the same rule: the layout held the box inside `axes`,
   // and the clip keeps the half pixel the rasteriser puts outside the box's fractional edges.
+  //
+  // The dash rides on the same segment: `v_along` is measured from the axes edge the line is
+  // staged from, so no geometry is added and the phase does not depend on the transform.
   const Mapping data = mapping(plot, 0u, 0u);
   if (crosshair.vertical.w > 0.0f) {
     const f64 x = plot.x_transform(0u).to_data_x(
         static_cast<f64>(crosshair.vertical.x + 0.5f * crosshair.vertical.w));
     const Range y_range = plot.y_range(0u);
     stage_segment(x, y_range.min, x, y_range.max, plot.x_axis(0u).log, plot.y_axis(0u).log);
-    flush_lines(data, style_.crosshair, stroke);
+    flush_lines(data, style_.crosshair, stroke, crosshair.dash);
   }
   if (crosshair.horizontal.h > 0.0f) {
     const f64 y = plot.y_transform(0u).to_data_y(
         static_cast<f64>(crosshair.horizontal.y + 0.5f * crosshair.horizontal.h));
     const Range x_range = plot.x_range(0u);
     stage_segment(x_range.min, y, x_range.max, y, plot.x_axis(0u).log, plot.y_axis(0u).log);
-    flush_lines(data, style_.crosshair, stroke);
+    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

src/render/shaders.cpp

@@ -18,7 +18,12 @@ uniform vec2 u_viewport;  // render target size, pixels
 uniform float u_width;    // stroke width, pixels
 
 out float v_dist;   // signed distance to the segment axis, pixels
-out float v_along;  // distance travelled from a_p0, pixels
+// Distance travelled from a_p0, pixels -- and the dash's phase anchor. Both of the
+// crosshair's lines are staged from an *axes edge* (its vertical from `y_range.min`,
+// the axes' bottom; its horizontal from `x_range.min`, the axes' left), so a dash measured
+// from here is measured from a fixed edge of the rect: it does not crawl as the view pans
+// or zooms, and a stationary crosshair renders identically frame to frame.
+out float v_along;
 
 void main() {
   vec2 p0 = a_p0 * u_scale + u_offset;
@@ -53,13 +58,28 @@ void main() {
 
 const char* const kLineFS = R"(#version 330 core
 in float v_dist;   // signed distance to the segment axis, pixels
+in float v_along;  // distance travelled from a_p0, pixels
 
 uniform float u_width;  // stroke width, pixels
 uniform vec4 u_color;
+uniform vec2 u_dash;    // dash pattern in pixels: x = ink, y = gap; y <= 0 is solid
 
 out vec4 o_color;
 
 void main() {
+  // The dash (spec §17): a fragment-shader discard, never new geometry -- the vertices
+  // stay in data space and the pattern is one per-draw uniform. `v_along` interpolates the
+  // distance from `a_p0` across the quad in pixels, so `mod` measures the pattern along the
+  // line rather than in screen space, which is what keeps it anchored to the segment while
+  // the view pans. `y <= 0` is solid, so every other stroke in the library -- grid, traces,
+  // axis lines, regions, cursors, the legend border -- draws unchanged by this uniform's
+  // default.
+  if (u_dash.y > 0.0) {
+    float period = u_dash.x + u_dash.y;
+    if (period > 0.0 && mod(v_along, period) >= u_dash.x) {
+      discard;  // in the gap of the pattern
+    }
+  }
   float half_w = u_width * 0.5;
   float d = abs(v_dist);
   if (d > half_w + 1.0) {