Hi;
I have been trying to figure out a way of doing this for a while and I think I've finally got it! I'm sure the more experienced folks here can improve on the method, but at least I've got it to where it is functional.
The whole deal is explained (and worked out) in the thread:
http://jpgraph.intellit.nl/index.php/topic,4159.msg10935.html#msg10935
on the jpgraph site. (hope this link works...)