Graphing the difference of Inbound & Outbound traffic

[idleline]

I have certain devices that I graph that will always have higher Inbound traffic levels than Outbound. I use the default "Interface (bits/sec)" graph template currently. Running 0.8.7e RRD 1.3.

I would like to take the value of the traffic_out data source subtracted from the traffic_in data source and graph it. I've looked through the HOW TOs for CDEF hacks but none that make any sense to me. Any ideas on a good way to accomplish this?

[gandalf]

cdef=b,a,-
assuming, that the DEFs are named a and b, respectively
R.