I was able to come up with a solution (not very elegant but it is what it is).
in the dimension (bar not group - grouped by year) I used the following in the fixed number formula:
if( IsNull(GetFieldSelections(Store_Name)), 5, 6)
Which means that if there is no filter - only top 5 stores are shown, but if there is a store filter - a 6th store (the selected one) is shown as well (the other 5 are in gray since they are not selected)