snmpwalk cannot find any CPU-related OIDs on the SNMP of m0n0wall
1.2b6 (WRAP), the mailing-list archive also has no clues so I
experimented a little and put this into /etc/ifgraph.conf
Results are here: http://www.fpage.org/ifgraph/
It does give the same results as the Adobe SVG Viewer, you can easily
adjust it for MRTG.