I've created a report that shows all invoices for a given department and groups them by client.  This allows sales reps to see which clients have purchased the most from that department.

I would like to sort the report in descending order by which clients have purchased the most.  But when I add that sorting order to the report it only sorts on the individual invoice, not the overall amount.  So the client with the largest invoice ends up on top not the client with the highest overall amount.

This seems like a relatively simple request, am I missing something?  How can I sort a report by amount when the results are grouped by client?