Install Plugin

$ curl -o /usr/share/munin/plugins/bitcoind_ https://raw.githubusercontent.com/MorbZ/munin-bitcoind/master/bitcoind_
$ chmod +x /usr/share/munin/plugins/bitcoind_

Create file /etc/munin/plugin-conf.d/bitcoind with contents:

[bitcoind_*]
user root

$ mkdir /.bitcoin
$ cp ~/.bitcoin/bitcoin.conf /.bitcoin

Setup Graphs

Number of connections:
$ ln -s /usr/share/munin/plugins/bitcoind_ /etc/munin/plugins/bitcoind_connections

Block count:
$ ln -s /usr/share/munin/plugins/bitcoind_ /etc/munin/plugins/bitcoind_blocks

Difficulty:
$ ln -s /usr/share/munin/plugins/bitcoind_ /etc/munin/plugins/bitcoind_difficulty

Don't forget to restart Munin after changing plugins:
$ service munin-node restart