feat(plot): the figure crosshair marks the hovered row, and the stroke is thin, dashed and configurable

src/plot/figure.cpp

@@ -449,6 +449,22 @@ void Figure::set_crosshair(f64 x, bool active) {
   for (Plot& row : plots_) row.set_crosshair(x, active);
 }
 
+void Figure::set_crosshair(f64 x, u32 hovered_row, f64 y_in_row, bool active) {
+  // The shared x first, on every row -- the same state the single-coordinate overload leaves,
+  // so the vertical lines and every row's readout are placed by one rule whether or not the
+  // pointer's row is known. A bad x is refused by the row call, which changes nothing, so the
+  // y below must not run either or the hover would be half-placed.
+  if (active && !std::isfinite(x)) return;
+  for (Plot& row : plots_) row.set_crosshair(x, active);
+  // Then the pointer row's own y: the *only* row that gets a horizontal line, because the y is
+  // a value on that row's scale and no other row's. A row that does not exist (the pointer is in
+  // the gap between rows) leaves every row vertical-only, which is the honest answer -- there is
+  // no row under the pointer to mark -- and a non-finite y leaves the row vertical-only rather
+  // than clearing a hover the pointer still has.
+  if (!active || hovered_row >= plots_.size()) return;
+  plots_[hovered_row].set_crosshair(x, y_in_row, true);
+}
+
 void Figure::clear_crosshair() {
   for (Plot& row : plots_) row.clear_crosshair();
 }