Rolling summary of # of unique accounts that have purchased in the past 12 months - Custom Object

I'm Running a report that currently shows me 

The number of unique account names that have made purchases each month - this works (see chart below). So i can see in March 2019 we had 485 customers buy from us. We know they have bought from us that month because we have an invoice date field.

I now want to show a rolling figure answers this question

QUESTION : How many customers do we have that have bought from us in the past 12 months? I want to be able to see data on any given month

eg In March we had 1300 unique customers(unique account name) purchase from us in the past 12 months, in Feb we had 1200 customers that purchased from us in the past 12 months.

I think I need to create a summary formula of some kind but have never done this before. A step by step of how to do this would be greatly appreciated.

Thanks everyone!

#Reports & Dashboards #Rollup Summary Field #Current Count

Here are screen shots of the data

Report Object: Accounts 

Account Name Field - shown as "Unique Count"