[UnderHost]

Fix for Munin installed by cPanel/WHM
[*] Determine where is mysqladmin program located with the one of the following command:

which mysqladmin
type mysqladmin
locate mysqladmin

If should return a path to mysqladmin which looks similar or the same to /usr/bin/mysqladmin.
[*] Edit the /etc/munin/plugin-conf.d/cpanel.conf Munin and modify the [mysql*] to look like below:

[mysql*]
user root
group wheel
env.mysqladmin /usr/bin/mysqladmin
env.mysqlopts --defaults-extra-file=/root/.my.cnf

Ensure the env.mysqladmin value is path returned by step above. Also ensure that /root/.my.cnf contains correct root password to the MySQL databases.
[*] Then restart Munin service:

/etc/init.d/munin-node restart

[*] Wait a while for MySQL statistics to be gathered, and check the graphs again.

Fix for Host or Server without cPanel

Simply add the “env.mysqladmin /usr/bin/mysqladmin” (ensure that the path is correct according to your system) to the /etc/munin/plugin-conf.d/munin-node file will do the trick, according to Munin FAQ.