What causes a parameter parameter error in my SSRS diagram?

Question

Why am I getting this error in my chart? ( Image chart )

I use this expression in a chart:

Series:

=Sum(Fields!Mins_Att.Value)/Sum(Fields!Mins_Poss.Value)

Series 1: 

=Sum(Fields!Mins_Att.Value, 
         "Chart2_CategoryGroup2")/Sum(Fields!Mins_Poss.Value, "Chart2_CategoryGroup2")

and I get this error:

The expression Y for the diagram has a scope parameter that is not valid for an aggregate function. The scope parameter must be set to a string constant that is equal to the name of the group, data region, or dataset name.

Answer 1

The scope "Chart2_CategoryGroup2" does not exist in the report.