Sorry I know there are loads of discussion about this topic, but I am still stuck on how to do this.

I'm currently trying to count how many distinct users (by User ID) are in my data  - but show this as a running total per month.

In the attachment, the blue line show distinct User ID for each month - this is correct.

But I want the orange line to show a running count of distinct User IDs - not sum each month (as the line is showing now).  I want the distinct count to exclude any ID's which are also included in a previous month.

thanks for your help