When you have some OpenVPN Roadwarriors configured in your IPFire, some connection data will be stored through the collectd daemon. For each connection the traffic data is stored in RRD files which are represented in nice graphs for each OpenVPN Client.
You are able to see, when a client connected and how much traffic was generated. The data is selectable for Hour, Day, Week, Month and Year.
Older VPN graphs can be removed when obsolete by the following steps:
rm -i /var/log/rrd/collectd/localhost/openvpn*/*.rrd
-iwill prompt before every removal.