There are many ways to do this. I use software called Cacti that runs on Linux. I think they have a Windows port for it as well. Everything runs from a browser and you can do things like graph port traffic, 95th percentile, total transfer, etc.
Cacti can graphy any SNMP data if you have the correct data/graph/host templates for it. Depending on how much you are graphing, you might want a dedicated box to do this. http://cactiez.cactiusers.org/
has an ISO that will install Linux and pre-configure Cacti for you. Very easy for newbies.