Have a look at http://www2.arnes.si/~gljsentvid10/95p1.html there's three links to the bottom of the page. There's a script you can run, an older version of mrtg and a patch to mrtg. It would depend on how you want to maintain it in the future, if your MRTG is installed as a package by the distro the script is probably the most maintainable option.