I’m using below expression in line chart to show year on year trend line. Where as the January data for the current year is including sums from previous years,
I tend not to use the RangeSum function, as you don't get full control. Instead I build an accumulated dimension into the load script. In your case you would have a table that links January to itself, February to itself and January etc.
This blog post talks about this technique, you will need to adapt it to your scenario though:
