How to average the values when doing multiple selection from a filter

I have a block on my task. Can you please help me to solve it?

I create a Tachometer gauge, and I want to display KPI value in one month if I select a month from a filter. That's OK for single month selection. But if I select several months, I want it show me the average value of these months, actually it seems to show me total value. How can I get average value?

 

Much appreciate if you can help.