This may be helpful for you,
Mean while, Can you please explain what your'e looking for,with some sample data if possible.
This is a neat D3 one which could be built into an extension
I wrote a tutorial series on how to create D3 extensions if you feel like some coding!
Emma , Extension object is the only solution for this. I had a similar requirement but finally settled using concentric pie charts for solution.(Screenshot below).
The problem with this is there is a limit to the number of different values(groups) that the pie can show.