Lizzy, it would probably be most helpful on this one if you can attach a QVW with the data model and some data and your chart etc., so folks can see the entire picture. Trying to solve these without that can be tricky. I suspect you will be able to get some responses if you can do that.
The best I have otherwise would be to search in the Design Blog area:
That is the base URL, you can search across the hundreds of posts there to find any useful examples etc... Hopefully that may get you what you need if you cannot post the QVW. There are quite a few posts on AGGR etc...
To help users find verified answers, please do not forget to use the "Accept as Solution" button on any post(s) that helped you resolve your problem or question. I now work a compressed schedule, Tuesday, Wednesday and Thursday, so those will be the days I will reply to any follow-up posts.