Bandwidth usage

I'm not sure if endian has this information stored somewhere or if theres a little hacking involved, but i would like to know how much bandwidth was consumed on my gateway/firewall per month.
What do i have to do to obtain this information from endian.
I assume it stores all this data somewhere as users are able to query bandwidth speed using the traffic graph.
Example of what im looking for.
Jan 2007 - Used 4.5 Gigs
Feb 2007 - Used 7.3 Gigs
etc...
Any pointing into the correct direction, much appreciated