This is a horribly manual way, but the only way I can think is is to have a report at the end of each month that has the people whose last login date is that month automatically delivered to you. Save the file and then combine them whenever you need them. Once you merge the dat, you can do a pivot chart on usage.
We are looking at doing an API call every night that is to capture who logged in that day and then we have the info in our data warehouse in SQL.
