feat(plot): crosshair with an interpolated per-signal hover readout

src/plot/figure.cpp

@@ -410,4 +410,18 @@ const FigureLayout& Figure::layout() {
   return layout_;
 }
 
+// ----------------------------------------------------------------- crosshair
+
+void Figure::set_crosshair(f64 x, bool active) {
+  // One x, every row: the readouts are synced because they are told the same coordinate, not
+  // because a renderer copies one row's to the others. `Plot::set_crosshair()` refuses a
+  // coordinate that is not a number, so a garbage value leaves every row's crosshair as it
+  // was rather than clearing some rows and not others.
+  for (Plot& row : plots_) row.set_crosshair(x, active);
+}
+
+void Figure::clear_crosshair() {
+  for (Plot& row : plots_) row.clear_crosshair();
+}
+
 }  // namespace plot