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

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