The best thing to do would be to install MRTG locally. You won't be able to log each individual service, i don't think. This way, you can monitor your server as a whole and see exactly how much bw your server is consuming. As for how cpanal logging, I believe the stats in whm monitor all the bw travelling over the domain so it also does not monitor each individual service by giving stats of each service (did that make sense?
Also, if you are not running cpanel, I suggest using webalizer to monitor each domain. It has pretty graphs that is fairly accurate. You can find out more info about webalizer here: