fix(input): clamp a dragged ruler on an ordered y range and share the orientation test

include/plot/plot/cursor.hpp

@@ -66,6 +66,17 @@ struct Cursor {
   u32 axis_x = 0;  ///< the x axis `x` is in. Dead for `Horizontal`.
   u32 axis_y = 0;  ///< the y axis `y` is in. Dead for `Vertical`.
   bool visible = true;
+
+  /// True when this is a horizontal ruler (spans the axes' width, reads its `y`). The one
+  /// place the orientation becomes a branch, so every consumer takes the same decision and
+  /// the two orientations cannot drift apart as new ones are added.
+  bool is_horizontal() const noexcept { return orientation == CursorOrientation::Horizontal; }
+
+  /// The live data coordinate: `y` for a horizontal ruler, `x` for a vertical marker.
+  f64 coordinate() const noexcept { return is_horizontal() ? y : x; }
+
+  /// The axis `coordinate()` is in: `axis_y` for a horizontal ruler, `axis_x` for a vertical.
+  u32 coordinate_axis() const noexcept { return is_horizontal() ? axis_y : axis_x; }
 };
 
 }  // namespace plot