Thanks for your reply!
Unfortunately the solution is not exactly as I had hoped yet.
I would like to have (a single calculated) field that could serve as the reference line (with same value) for both graphs so that the maximum of the axes is always aligned. So I would like to add a reference line to the left graph as well.
I think your approach is going in the right direction but I can't get the formula right so that it serves as the same reference value for both graphs. I don't quite get the calculation that is being performed by the formula you proposed. If I try to add it as a reference line to the other graph, it gives me a different number.
Any thoughts on this?
Thanks for the effort you are putting in!