Monitoring Bandwidth (transfer) in GB / MB

I have been trying to search for a way to monitor customers bandwidth
transfer usage by GB. I have not found any method to do this with rrdtool or
mrtg. I could definitely be missing an industry term that I should search
for since google does not return anything useful.

Does anyone have a list or recommendation for such a tool?

Bytes/s * (86400 * $days_in_month).

RRDTool is *very* flexible when it comes to displaying anything. Usually it does autosizing. Try
something lile

rrdtool graph $output --units-exponent 9 ...

If this doesn't help try

Regards, Arnold