yes, I'll think about it.
Anyway, I'll go another way. These charts are just way too quirky, they already caused me enough of a headache. They do indeed display the correct values using a set_expression, you just must not make any selections in the multi_selection_box then. Since all that is only relevant for the reports_to_be_printed, that's a good way to go.
I'll mark this as solved then. It would take up too much time to try and really get behind this.