Below is the link for the user aggregation options but the requirement you are looking for going to be enormous if you store in the database or the file system is required in Terabytes if you aggregate user data on demand.
Like babar say: we have customers with storage period for raw data about a month and it blows up the database enormous. It has also side effects on CAS reporting functionalities at this customer.
This was the only way to get the number of users for a month.
I think I would do a daily, weekly or monthly export of data to a 3rd party tool and do trending based on this exported data