I recently had a situation where I installed a munin-node on a server, and the first readings it put out were garbled. I mean, that were 10^250 or some such ridiculous value. This was messing up the graphs for the day...and then the week and so on. It also messed up the averages.

I figured there must be an out-lier that was causing this to happen, but I was stuck with the issue of how to remove this single value (or string of values). Munin stores it's values in an RRD stored in /var/lib/munin on Debian. What I had to do was to export each database for the graph I was concerned about - the memory graph has several elements, each one a separate database - into XML. Then edit the XML (just replacing the out-lier with NaN does the trick). Then reimport the XML, forcing overwriting of the existing data to make sure I replace the old value.

cd /var/lib/munin
rrdtool dump server-memory-swap-d.rrd > /home/user/server-memory-swap-d.xml
rrdtool restore /home/user/server-memory-swap-d.xml server-memory-swap-d.rrd -f