it seems this is simply not possible. I can stack columns only in a pure barchart, but then i cannot show lines.
An alternative might be to define the "column offset" (formula properties) so that this is equal to the value I would show in the other formula.
That looks a bit weird, but it works fine - for the individual values.
The diagram, however, can also show the cumulative YTD-values - the difference is just a checkbox on the "expression" tab. How could I achieve the same effect in an expression (in the formula properties)?`
Thanks a lot!