Count unique number of attendees of events within a month

Hi Guys,

I have three tables:
{Attendee table}
{Bridge table}
{Event table}

The {Bridge table} is where we check in attendees to an event by using two fields:
Select the event from a linked record field (links to {Event Table})
Select the attendees from a linked record field (links to ‘{Attendee table}’)
The only record we create during this process is within the {Bridge table}, the records in {Attendee table} and {Event table} already exist.

In the {Event table} we have the a date field with an additional formula field calculating the month the event occurred.

My aim is to create a chart block to count the number of attendees who attended at least one event during a given month, as an attendee can attended one or more events per month (this is easy enough to calculate with blocks).

I cannot figure out how to do this, any ideas? Have I missed something super obvious?

Thanks.