+ /** return @param y as a floating point fraction of the overall
+ * height of the rectangle. @param y is in canvas coordinate space.
+ *
+ * A value of zero indicates that y is at the bottom of the
+ * rectangle; a value of 1 indicates that y is at the top.
+ *
+ * Will return zero if there is no bounding box or if y
+ * is outside the bounding box.
+ */
+ double vertical_fraction (double y) const;
+