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;