sorry to jump in so late, but I noticed this thread and wanted to help explain something dwhitfield mentioned in his recent post, in case you're not familiar everything.
Nagios XI is paid version, but it also offers a *much* simpler (and free) way to try out things like monitoring and graphing ports with Nagios.
You can get it as a CentOS VM that you can run in VirtualBox:
https://support.nagios.com/kb/article.php?id=289
Like he said, it is free for 7 or less devices (hosts) too.
In XI, you just run the Switch&Router wizard, pick the ports to monitor and boooom- monitoring and graphs.
It will help you see the potential of Nagios & MRTG, and all you'll probably have to do is spin up a VM and follow the steps in a wizard to see results
It definitely helped me learn how Nagios Core all the related components and concepts worked, maybe it does the same for you!