I had this same issue, but only after I noticed that zenhub wasn't working after the upgrade from 4.2.3 to 4.2.4. To fix my zenhub issue, I followed the information in this post....
http://community.zenoss.org/message/73697#73697
... and that got me thinking that's probably what's happening with my graphing data as well. I followed the same procedure with the $ZENHOME/etc/zenrender.conf file. After modifying and restarting Zensoss, all graphs are working great now.