From the Munin FAQ
Q: Why are the graphs for the MySQL plugin blank?
This is due to a bug in a Perl library Munin uses which causes $PATH to be lost. This again causes the plugin to not find the mysqladmin program which it needs to retrive the numbers the needed in the graphs. The solution is to hardcode the path of the program.
First locate the mysqladmin program. On most systems, the command which mysqladmin, type mysqladmin or locate mysqladmin will help you. When you find the path, enter that path in /etc/munin/plugin-conf.d/munin-node.
This is how it might look:
darkstar:~# which mysqladmin
Then, under the [mysql*] section identifier in /etc/munin/plugin-conf.d/munin-node, add the following line:
Found in the Munin Docs