Different level of detail matrix rows

I have two fields for my customers. One with the customer name and one with the group name.

E.G. Matrix rows:

       Field: Customer Name, value: Retailer_1, Retailer_2 etc.

       Field: Group, value: City_1, City_2 etc.

I can expand and collapse the group field for all the fields in the Group, however I want to expand one group to the customer name value and leave the other groups as a group:

E.G.

     Matrix Rows:

     Retailer_1 

     Retailer_2  

     City_1

     City_2

Does anybody have any idea how to do that?