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

src/plot/plot.cpp

@@ -344,6 +344,10 @@ void Plot::set_y_axis_origin_from_range(u32 axis, bool on) {
 
 void Plot::set_x_note(std::string text) { x_note_ = std::move(text); }
 
+void Plot::set_invalid_spans(std::span<const std::pair<f64, f64>> spans) {
+  invalid_spans_.assign(spans.begin(), spans.end());
+}
+
 f64 Plot::x_axis_origin(u32 axis) const {
   return axis < x_axes_.size() ? x_axes_[axis].origin : 0.0;
 }
@@ -1091,6 +1095,7 @@ const PlotLayout& Plot::layout() {
   // measured from the *placed* live windows, which is what makes a region's data
   // interval map to the pixel columns the frame will actually use.
   build_regions();
+  build_invalid_spans();
   build_legend();
   // The crosshair last: its panel is fitted into `axes` and its lines are held inside it, and
   // its rows are the sample values nearest a data x -- so it wants the transforms the
@@ -1358,6 +1363,41 @@ void Plot::build_regions() {
   }
 }
 
+void Plot::build_invalid_spans() {
+  layout_.invalid_spans.clear();
+  if (invalid_spans_.empty()) return;
+
+  const Rect& axes = layout_.axes;
+  // The primary x axis' transform, taken here for `build_regions()`' reason: after the live
+  // windows have been placed and the strips have settled, so these are the columns the frame
+  // draws with and a band is anchored to the data by construction rather than by a second
+  // mapping that would have to be kept in step with this one.
+  const Transform tx = x_transform(0u);
+  // A hole can be narrower than a column -- one missing sample in a window of hundreds of
+  // thousands -- and a fill rounded to zero width would draw nothing, losing exactly the case
+  // the band is most needed for. One device pixel is the floor.
+  const f32 min_w = std::max(1.0f, style_.device(1.0f));
+
+  for (const std::pair<f64, f64>& span : invalid_spans_) {
+    const f64 a = std::min(span.first, span.second);
+    const f64 b = std::max(span.first, span.second);
+    if (!std::isfinite(a) || !std::isfinite(b)) continue;
+    const f32 lo = static_cast<f32>(tx.to_screen_x(a));
+    const f32 hi = static_cast<f32>(tx.to_screen_x(b));
+    // Scrolled out of the window: nothing to shade. Tested before the floor is applied, so a
+    // span off the left edge is dropped rather than pinned to it as a one-pixel band that
+    // accuses the samples at the window's start.
+    if (hi < axes.x || lo > axes.right()) continue;
+    const f32 first = std::clamp(lo, axes.x, axes.right());
+    const f32 last = std::clamp(hi, axes.x, axes.right());
+    const f32 w = std::min(std::max(last - first, min_w), axes.w);
+    // Widening a band at the right edge would run it over the axis line, so the floor is taken
+    // out of the left instead: the span's own edge is kept wherever there is room for it.
+    const f32 x = std::clamp(first, axes.x, axes.right() - w);
+    layout_.invalid_spans.push_back(Rect{x, axes.y, w, axes.h});
+  }
+}
+
 // --- crosshair (spec §15) --------------------------------------------------
 
 CrosshairSpan crosshair_span(const data::Series& series, f64 x) noexcept {

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);