Returns the index of the subplot that contains the specified
(x, y) point (the "source" point). The source point will usually
come from a mouse click on a
org.jfree.chart.ChartPanel ,
and this method is then used to determine the subplot that
contains the source point.
Parameters: source - the source point (in Java2D space, null notpermitted). The subplot index (or -1 if no subplot contains source).