This plots four series over the time selected: Sent reminders, Due reminders, Sent customers, Due customers.

The customer lines are also based on reminders, but aggregate. This should give you an idea of how much you're paying in postage (if you're doing that).

One of these may be a better leading indicator for revenue than the other... but I haven't experimented with that.

I'd appreciate if someone can check the SQL logic, because it got a little complicated in there.