fixed user per group graph

The glitch in the graph was caused by a relatively long time distance between the two last data values from mail_domains.csv.

The graph now only uses the data from aai_userstats.csv, instead of using combined data from aai_userstats.csv and mail_domains.csv.

aai_userstats.csv now has more up-to-date and more regular data, which makes the use of mail_domains.csv unnecessary.

#24 (closed) is the original issue, discussing the need of this plot and how to achieve it.