How to order a report by sum?

Hello!

I'm new in AX, so I apologize if I can't express very well.

I need to build a report (in AX 2009) which shows how much was bought from each vendor in a certain period. Ex: if I want to see january data and if we have 2 purchase orders with the vendor X, one of them costing 1,000.00 and the other one costing 2,500.00, the report would show the name of vendor X in a column and the value 3,500.00 in another one.

It's ok until there, the report is getting all data I need. But I was asked to make the report be ordered by the values (like if, in a "standard" query", I added "Order By Sum(PurchTable.LineAmount)").

I put the field PurchLine.LineAmount (in which the sum was done) under the "Order By" node in AOT, to no avail... I'm looking for the solution for some time without success. Can somebody help me, please?

Thanks in advance!
Luiz Bezerra