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

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;
@@ -52,14 +57,29 @@ void main() {
 )";
 
 const char* const kLineFS = R"(#version 330 core
-in float v_dist;  // signed distance to the segment axis, pixels
+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) {