monitoring traffic volume on a lan port

I have one Tomato (1.28) monitor traffic on the wan port. If find it highly useful. For another location, with the Tomato router, also 1.28, serving as an access point only, I find I would like to monitor the lan port that supplies the wifi access. Can someone suggest how that might be achieved?

I've searched around quite a bit. If my answer exists, it is lost to me in the noise.