David: Unfortunately, the drawback to using custom or calculated groupings is that you can't use the chart features of the report. A possible workaround is to create a calculated field within a custom form and then use that calulated field for your grouping. You'll just need to make sure that the custom form is attached to the object that you are reporting on.
Example Use Case I'm currently using this method to group a task report by a calculated field called "Timeliness." The report displays a certain template task and the grouping is defined from a calculated field in a custom form. When I first started developing this report, I assumed that I could calculate the "timeliness" values by using a calculated grouping. This is when I realized that I would lose charting functionality. To get around that, I created a custom form with a calculated field. This calculated field establishes the "timeliness" value (e.g. On Time, 1 Day Late, 2 Days Late, etc.). Once that calculated field was created, I was able to use that field in the grouping for the report and use the charts.
I hope this helps!
Becka
