For MRTG+qmail there are many scripts, but I couldn't find one I liked for Munin.
Since munin is modular, it's easy to fix, of course.
First, get the excellent qmrtg which gives you great, quick, awesome multilog-crunching tools.
Then, check the cods, check the munin plugin docs, and it's pretty much a 20-line thingie. In shell.
I would post it, but my blogging tool hates shell code :-)
And you get your graphs. I am probably going to write a collection of these and publish it somewhere.