How to group values of the same field and display timechart counts for each group month over month?

I first need to group values of the same field... Group1 (values match A1, A2, A3,...) Group2 (values match B1, B2, B3,...) Group3 (values match C1, C2, C3,...) ...then, I need to display the counts for each group (Group1, Group2, Group3) month-over-month. Thanks for any assistance! Trista