This is actually quite simple. Look in the MRTG directory in the source tree.
Do this in the same way as you do "per host" monitoring except check your web server chains instead of the host chains. Better support for this will be in future versions.
There are actually several reasons to use rChains/ipchains rather than using a managed hub/switch.
Enable the hostrules() function in the start() function (/etc/sysconfig/firewall/functions).
Note: in old versions of rChains, this code can be found near the end of the lanaccess() function (/etc/sysconfig/firewall/functions or /etc/rc.d/rc.firewall.functions).
Perl lends itself to doing what is needed to make rChains very robust and much easier to maintain. However, I should note that I like having rChains written in bash. Therefore, I stress the "partially written in Perl" part.