When you 'select' a value in an expression cell then you don't select that expression value, but you select all the dimension values used to calculate the expression value. That's one reason why you get that selection you see. There's no solution for that. You cannot select expression values.
If you make selections in a chart, you are basically selecting in the dimension values your grid cell is grouped by.
In your case, you are using calculated dimensions based on CustID, hence you are making selections in CustID (even if you don't see it in the chart, since you are also using set analysis to ignore CustID selections.
Create a list box for CustID and you will see the effect when selecting in your chart cell.
Yes, Graphs will effect based from Data Model and Associative only, Have you seen the first image, You are selected two years then associative to All CustID. Would you explain more what you want to achieve so that we will show you different menthod