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

adapters/glfw/glfw_host.cpp

@@ -494,12 +494,15 @@ void Host::sync_figure_crosshair(const std::vector<Rect>& rows) noexcept {
     if (hovered >= row_count) return;
   }
 
-  // Applied to the whole figure: one x, every row. A row whose controller has *no* crosshair (the
-  // pointer left its axes, which is the only thing that clears one) clears the figure's, so the
-  // rows cannot be left showing a hover the pointer has abandoned.
+  // Applied to the whole figure: one x, every row; and the pointer row's **own** y, only to that
+  // row -- so the row under the pointer gets the one horizontal line, and no row ever shows
+  // another row's y (spec §17). The y is the row's own data coordinate, placed by its controller
+  // from the pointer's pixel row, and it is never spread to a sibling. A row whose controller has
+  // *no* crosshair (the pointer left its axes, which is the only thing that clears one) clears the
+  // figure's, so the rows cannot be left showing a hover the pointer has abandoned.
   const Plot& row = figure_->plot(hovered);
   if (row.crosshair().active) {
-    figure_->set_crosshair(row.crosshair().x, true);
+    figure_->set_crosshair(row.crosshair().x, hovered, row.crosshair().y, true);
     spread_x_ = row.crosshair().x;
     spread_active_ = true;
   } else {