[mrtg] separating WAN traffic from LAN traffic

[Joel Uckelman]

I have an interface for which I'd like to graph only the traffic to and from
the WAN, and ignore the traffic to and from the LAN. Graphing the total (LAN
+ WAN) traffic is useless for me, as I don't care about traffic on the LAN.
I only have SNMP access to my own box, not the router it sits behind, so
asking the router for this information is not possible. I do know exactly
which IPs are on the LAN, however, so I can determine locally what traffic
will go out to the WAN and what traffic is coming in form the WAN.

Is there some simple way to graph just the WAN traffic, given my constraints?
I've searched already for solutions to this problem---which seems like it
should be a common one---and turned up nothing.

-- 
J.