Usually this happens when munin plugin is not able to connect to mysql and grab statistica data. Here is what I do to fix this problem:
cp /root/.my.cnf /home/munin/.my.cnf
chown munin.munin /home/munin/.my.cnf
then in the file /home/munin/.my.cnf I change the word [client] to [mysqladmin] and restart munin:
Yes, I know, that giving munin root privileges is a bad idea. On the other hand noone will login with that user, but if you are concerned - you could add special user to mysql with permission to run the following command:
and of course put that login/pass to /home/munin/.my.cnf.
I hope that helps!