Overhead Transparencies in Bar Charts

Just started using JChart and it is very intuitive and easy-to-use. Not
sure if this is a bug or a missing feature but I want to have multiple
bar charts overlayed. I am sharing the same X and Y axis scales which
simplifies things. When trying the "combo" thing it seems that only the
"last" series added is displayed. I am using PNG with translucent colors
(alpha channel) to produce the effect of transparencies using an
overhead projector where you lay one slide on top of the other. Anybody
bean there done that