Report on number of defects in Filed Against

I've recently started experimenting with reports. All the template ones I have seem to report based on Team Areas. I find this almost 100% useless unfortunately - what I need to do is report on Categories/Filed Against. So for example I want to see the number of defects (by severity) over a time period, where each defect is Filed Against a certain set of categories (instead of opened by a certain team of people).

Has anyone done anything like this? I'm having a hard time building a report to do it - I can't get the Data Source stuff right to even pull the information I need, never mind build a visualization of it.

Thanks for any help - much appreciated.
Richard