feat: a span declared invalid is shaded behind the data it is missing from

src/render/renderer.cpp

@@ -516,6 +516,9 @@ void Renderer::draw_plot(Plot& plot, const PlotLayout& layout) {
   // against the line passes would not change a pixel.)
   set_clip(layout.axes);
   draw_grid(plot);
+  // The holes over the grid and under the data: a band is chrome of the *background*, not an
+  // overlay, so everything a frame draws inside the axes is drawn on top of it.
+  draw_invalid_spans(layout);
   clear_clip();
   draw_axis_lines(plot);
   set_clip(layout.axes);
@@ -1728,6 +1731,12 @@ void Renderer::draw_cursor_label(const Plot& plot, const PlotLayout& layout, con
   }
 }
 
+void Renderer::draw_invalid_spans(const PlotLayout& layout) {
+  for (const Rect& band : layout.invalid_spans) {
+    if (band.w > 0.0f) fill_pixels(band, style_.invalid_fill);
+  }
+}
+
 void Renderer::draw_regions(const Plot& plot, const PlotLayout& layout) {
   const HotHandles hot = plot.hot_handles();
   const f32 edge_w = style_.device(style_.line_width_px);