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

include/plot/render/renderer.hpp

@@ -513,6 +513,12 @@ class Renderer {
   void draw_plot(Plot& plot, const PlotLayout& layout);
 
   void draw_grid(const Plot& plot);
+  /// `Plot::invalid_spans()`, as `PlotLayout::invalid_spans` measured them: one fill per span in
+  /// `style_.invalid_fill`. Over the grid and under everything else inside the axes, so a hole
+  /// reads as a property of the plot's background -- the grid shows through it and a trace that
+  /// does have data in those columns is drawn on top, which is what distinguishes a reserved
+  /// hole from a trace that has gone quiet.
+  void draw_invalid_spans(const PlotLayout& layout);
   void draw_axis_lines(const Plot& plot);
   /// The chrome helpers take the frame's layout: `Plot::layout()` recomputes the
   /// ticks, the strips and the grid, so it is computed once, in `draw()`, and handed