You can consider 2 other ways that are not using SET:
1. In LOAD script create another column which contains the precedent month value.
2. Create a secondary disconected TIME table and use:
" =SUM( IF( Date = Dis_Date, Value ) )" for current month
" =SUM( IF( Date = addmonth(Dis_Date,-1), Value ) )" for last month
thanks for all the good ideas
But I haven't found the solution I could use yet:
1) I can't use above() because it is only possible when the previous column is also visible in the table (which is not the case for the month 11)
2) The approach using the max() - function works if I only have two months, but if I want to display 3 or more months it won't work anymore.
3) I can't us the Load - Skript because the analysis is quite dynamic and I can't forsee all selections.
So is there any way that I could use a set that references a dimension value - the month - of the current row?