I don't believe it is possible using the cfchart tag. If you
c:\coldfusion8\charting there is a .bat file called
webcharts.bat as well as a userguide. If you know your way around
java a bit you can bypass the cfchart tag entirely and use the full
power of the webcharts api yourself.
I am currently doing something similar using an opensource
charting library and I think I may have to try and provide that
feature in the custom tag I am building.