How to group by in NL

Hi all,

I have created quite a few jetreports at our company, but I have not found any sollution for the following problem.
The main problem is that I don't know how to create a 'group by' in jetreports.

What I want to create is a matrix showing the sales for the entire year :
Rows : selected customer / customer articles
Columns : month
Result : total invoiced

I can get the correct results and columns without any problem.
My problem is displaying the customer and the customer articles.
I have no seperate table containing the articles for a specific customer !
Also an article can be sold to multiple customers.

So I try to use the articles from the "value entry"-table for that customer.
But if I do it this way, I get multiple line for the same article, one for each invoice that was made.

So my question is basicly :
How can I create a NL-funcion that only shows 1 instance of each article for that specific customer ?

Thanks in advance.