After installing a new Gentoo server running rrdtool-1.3.8 and munin-1.3.4 I received many mails from munin's cronjob, complaining the following
(process:22748): Pango-WARNING **: Invalid UTF-8 string passed to pango_layout_set_text()

A little research showed up something like the following Bug#494547: munin cron job complains about pango invalid utf-8 string.

The current version should work in any of the following two cases:

• the script's encoding matches the locale settings
• the script is encoded in UTF-8

A quick solution is makeing munin-graph to change munin's plugins texts to UTF-8.
A suitable patch can be found here: 380-munin-graph-utf8.patch

Apply the patch

patch -i /tmp/munin_1.3.4-r1_utf8.patch