Wanted to add to the post and thank Deepak, who ended my full-day search through the community for the solution to a similar problem. As mine was a bit more complex - I could not work with a Year Dimension in my Pivot Chart and had to get the "Year" information into the Quarter selection. I adapted the variable approach like this:
1. Created a List box based on my dates but used an expression to condense to Quarters, so to a user it would appear as a single-selection, whereas all days of the quarter are selected. =Year(Date)&'_Q'&ceil(month(Date)/3)
2. Defined a variable vYQ-1 based on the current date selection, calculated one quarter back (minus 92 days) and used Year Function: =Year(date(max(Date)-92,'DD.MM.YYYY'))
3. Defined a variable vQ-1 based on the current date selection, calculated one quarter back (minus 92 days) and used the function to get: =ceil(month(date(max(Date)-92,'DD.MM.YYYY'))/3)
4. Used Set Analysis in the chart to get the "Previous Quarter" from the current selection =Sum({1<Year={$(vYQ-1)},_Qtr={$(vQ-1)} >} Amount)
