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

include/plot/render/style.hpp

@@ -55,6 +55,13 @@ struct Style {
   Color axis_text = Color::hex(0xC8D0DC);
   Color cursor = Color::hex(0xFFB020);
   Color measure_fill = Color::hex(0xFFB020, 0.15f);
+  /// The shading over a span the host has declared invalid (`Plot::set_invalid_spans()`): the
+  /// axis furniture's own neutral grey at a low alpha, on the same model as `measure_fill`
+  /// above. Neutral and not a trace colour on purpose -- a band is the *absence* of data, and
+  /// one in a hue a trace could have would read as a signal rather than as a gap. The alpha is
+  /// low enough that the grid under it stays legible, which is what makes the band read as a
+  /// region of the plot rather than as a panel laid over it.
+  Color invalid_fill = Color::hex(0x8A94A6, 0.18f);
   /// The crosshair's colour: its two lines and the readout panel's border (spec §15). Markers are
   /// deliberately *not* this colour -- they carry each trace's own, because a marker says *which*
   /// trace and the line says *where*.