Hello!
For now exists new interesting chart type - TreeMap (look example here http://www.smartmoney.com/marketmap/?nav=hp_marketmap)
Whether there are plans to add TreeMap charts to JFreeChart?
It is not so hard, because here http://treemap.sourceforge.net/
exists free and open source library implementing TreeMaps. Unfortunately, this library not supporting for a long time.
You can read more about this chart type here
http://www.cs.umd.edu/hcil/treemap-history/
Thanks,
Andrew