Second expression ignoring second dimension value in combi chart

[curiousfellow]

I have a bar chart with the dimensions  period and year, and one expression.

I would like to add a second expression as a line. The expression does not show average or trend or something like that.

In that expression I want to ignore the year-dimension, so every period has the same value and shows a straight line in the chart. Is something like that possible ? Please see example below (blue lines should be straight)