feat(plot): horizontal rulers alongside the vertical cursors

include/plot/render/renderer.hpp

@@ -452,7 +452,19 @@ class Renderer {
   /// The data helpers take it too; they read the axes rect and the transforms it
   /// pins.
   void draw_traces(const Plot& plot, const PlotLayout& layout);
+  /// The cursors, both orientations: one stroke per cursor, placed at the coordinate its own
+  /// orientation owns (`Plot::cursor_screen_coordinate()`) and spanning the other axis' whole
+  /// range, in its own colour -- or in the accent at handle width while it is hovered or
+  /// grabbed, which is the same cue a region's hot bound gets. A cursor at the far edge of
+  /// its range is skipped rather than drawn as half a stroke over the strip beside it.
   void draw_cursors(const Plot& plot, const PlotLayout& layout);
+  /// One cursor's label: the caller's caption for either orientation, and for a horizontal
+  /// ruler the value itself, written in its y axis' own tick format (`Axis::format_value()`)
+  /// -- the number a ruler exists to be read off. A vertical cursor's caption starts at the
+  /// top of the axes beside its marker; a ruler's text starts at the axes' left edge on its
+  /// own row, with the caption first on that line when there is one.
+  void draw_cursor_label(const Plot& plot, const PlotLayout& layout, const Cursor& cursor,
+                         f32 at);
   /// The regions: the fill, one bound per edge, the optional label. Every rect comes
   /// from `PlotLayout::regions` -- the same geometry the controller hit-tests -- so
   /// a region is drawn where it can be grabbed, and neither side derives the other's