This tracker item has the fix:  https://sourceforge.net/tracker/?func=detail&aid=3099189&group_id=227755&atid=1071634

Basically, if you use positive and negative numbers in the same bar on a graph, then pChart won't draw them correctly.  This patch fixes that.  It would be great to see this or equal functionality in the upcoming 2.0 release.  I'm happy to make the changes myself if someone can grant commit access. 

Thanks,

-Julian