Do a search on contribs.org for IOG - I did a howto for it on SME6.0, and it shows pretty much what you're after. IOG breaks it down into hourly/daily/monthly blocks.
If you use Knuddi's (swerts-knudsen's) MRTG script that will do the hard part of SNMP install and interface enumeration. All you really have to do then is look up the mrtg.cfg file his script creates, take note of the interface you want metrics of then plug that into the iog.cfg.
I might re-write that howto for SME7 and/or Knuddi's MRTG one day.