Include dimension values from selected month and previous month
The dimension 'Position' is made up of unique id numbers, each having a value between 0 and 1. This value can change by month. I want to compare values for each position for the selected month and the previous month. This is straightforward, except that position numbers that existed last month, but not in the selected month are not included.
How can I include positions that existed last month, but not this month?