How to do a double grouping for chart results

Hi. I want to create a chart with monthly results showing how many times something unique happened. For example we have users that came to the office sometimes. It could be one time in a month or multiply times. So I need to show how many unique users were at the office every month (doesn’t matter if the user was 1 time or multiply times it shows as one unique user)

the bubble manual is pretty good at explaining how to use the group by operator…you can set multiple aggregations and groupings to suit your needs