I think I'm not understanding your question. It would seem to me that you'd just have, say, Year as the dimension, and sum(Amount) as the expression. It would then be showing total amounts for each of the two years in the same diagram. If you wanted to compare monthly totals for the two years, then you'd have both year and month as dimensions.
I think I now have what I needed--in a way. As you said I use Year as dimension.
I then added two expressions ...
-- sum(Amount) * sum(if(Year=selected_Year1, 1, 0)
-- sum(Amount) * sum(if(Year=selected_Year2, 1, 0)
to make the diagramm show the two selected years only. But it looks abit "arount the corner" for me. Can it be done in a more elegant way?
PS: I also wonder why I must use "sum". I only have one amount per year.