Would underestimate player growth in the recent past and overestimate growth in the far past because people don't join clans immediately.
Wait, is 'join date' the date they joined the *clan*, or the date they joined *Warlight*?
If the latter, which is what I was thinking, then no it would not overestimate. It's an XY plot, and it's monotonic (in theory*), so each point stands on its own. You know their exact Warlight join date and their user ID. You just plot them out.
If the former, then I don't get the point of tracking that.
[* Based on your last comment/chart, looks like either the theory is broke, or somehow the data got messed up.]
Edited 12/26/2015 04:21:19