Report on development of list of recipients over time

Hi there! 

I have a list of recipient that contains inactive recipients. This list is updated once every day through a targeting workflow. 

I would like to see the development of the amount of inactive users over time so I can see the impact of our re-engagement flows as well. 

I have not yet succeeded with creating a report on how this list develops over time. Does anyone have experience with a similar task or any pointers to how this can be achieved? 

Right now I am thinking to make a SOAP call to get the amount from an external client and store that number every day so I can make everything externally, but that seems redundant if it can be done in ACC directly.