fix(plot,render,glfw): a hue-neutral crosshair, an em-dash placeholder, one layout per figure event, and a usable readout switch

src/render/renderer.cpp

@@ -1190,7 +1190,7 @@ void Renderer::draw_swatch(const Plot& plot, u32 trace, const Rect& box) {
 
 void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout) {
   const CrosshairLayout& crosshair = layout.crosshair;
-  if (!crosshair.active) return;
+  if (crosshair.empty()) return;
 
   const f32 stroke = style_.device(style_.line_width_px);
   // The two lines. Each is staged as one data-space segment at the *centre* of the stroke box
@@ -1231,7 +1231,7 @@ void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout)
 
 void Renderer::draw_crosshair_readout(const Plot& plot, const PlotLayout& layout) {
   const CrosshairLayout& crosshair = layout.crosshair;
-  if (!crosshair.active) return;
+  if (crosshair.empty()) return;
 
   // The readout: the same backplate a legend has (the shared `draw_readout_panel()`), with its
   // border in the crosshair's own colour -- which is what makes the panel read as part of the
@@ -1242,9 +1242,9 @@ void Renderer::draw_crosshair_readout(const Plot& plot, const PlotLayout& layout
     if (!row.listed) continue;
     draw_swatch(plot, row.trace, row.swatch);
     stage_text({row.label_box.x, row.label_box.y}, row.label.c_str());
-    // A row with no sample at this x has no value to write -- and it is *not* written as a
-    // zero or a dash: the box is empty and the number is simply absent (see CrosshairRow).
-    if (row.valid) stage_text({row.value_box.x, row.value_box.y}, row.value.c_str());
+    // The value is a number, or `kCrosshairNoValue` for a trace with no sample at this x (the plot
+    // put it there): either way there is a run to stage, so a row is never a blank cell.
+    stage_text({row.value_box.x, row.value_box.y}, row.value.c_str());
   }
 }