feat: translucent area fill under the in/out plot traces

Draw a translucent area from each level line down to the plot baseline so the
input and output traces read more clearly. The gain-reduction trace stays a
plain line (it hangs from the 0 dB line, where a fill would read oddly).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/editor/plot.rs

@@ -167,18 +167,32 @@ pub(super) fn draw(ui: &mut egui::Ui, meters: &Meters, state: &mut PlotState) {
     let c = state.selected.min(NUM_CHANNELS - 1);
     let (write, len) = (state.history.write, state.history.len);
     if len >= 2 {
-        let draw_series = |series: &[f32; PLOT_N], to_db: &dyn Fn(f32) -> f32, color: Color32| {
+        let draw_series = |series: &[f32; PLOT_N], to_db: &dyn Fn(f32) -> f32, color: Color32, fill: bool| {
             let mut pts = Vec::with_capacity(len);
             for k in 0..len {
                 let idx = (write + PLOT_N - len + k) % PLOT_N;
                 let pos = (PLOT_N - len + k) as f32 / (PLOT_N - 1) as f32; // newest hugs the right
                 pts.push(pos2(left + pos * width, y_for_db(to_db(series[idx]))));
             }
+            if fill {
+                // Translucent area from the line down to the bottom of the plot.
+                let mut area = Vec::with_capacity(pts.len() + 2);
+                area.push(pos2(pts[0].x, bottom));
+                area.extend_from_slice(&pts);
+                area.push(pos2(pts[pts.len() - 1].x, bottom));
+                let fill_col = Color32::from_rgba_unmultiplied(color.r(), color.g(), color.b(), 40);
+                p.add(egui::Shape::Path(egui::epaint::PathShape {
+                    points: area,
+                    closed: true,
+                    fill: fill_col,
+                    stroke: egui::epaint::PathStroke::NONE,
+                }));
+            }
             p.add(egui::Shape::line(pts, Stroke::new(1.5, color)));
         };
-        draw_series(&state.history.in_db[c], &|db| db, COLOR_IN);
+        draw_series(&state.history.in_db[c], &|db| db, COLOR_IN, true);
-        draw_series(&state.history.out_db[c], &|db| db, COLOR_OUT);
+        draw_series(&state.history.out_db[c], &|db| db, COLOR_OUT, true);
         // GR hangs from the 0 dB line: a reduction of X dB is drawn at the -X gridline.
-        draw_series(&state.history.gr_db[c], &|gr| -gr, COLOR_GR);
+        draw_series(&state.history.gr_db[c], &|gr| -gr, COLOR_GR, false);
     }
 }