Voila, you're good to go. If you're using a different brand of modem box, it may be covered in the contrib directory. If it's not, try snmpwalking one box and looking at the results. See if you can find where it's reporting connected lines. Now that you have a way to get data points into your graphing program, you're ready to automate.
Automation is good.
Unix variants have cron which allows you to automate stuff. We run new polling every five minutes for our graphs. This is frequent enough to provide fairly decent data and infrequent enough to avoid serious load on the systems we are polling. You could probably get by with doing it less frequently if you wanted. But if you want to graph more than two variables on the same plot, you are not going to be able to run mrtg.