I did install munin and at first my graphs looked like this on this image. So as you can see this looks like something is wrong here.
The solution was to modify my cron job to look like the following.
su -s /bin/ksh _munin -c 'LC_CTYPE=en_US.UTF-8 /usr/local/bin/munin-cron'
It seems that munin-cron does require a LC_CTYPE value, otherwise it generates without any text.