Install Plugin

$ curl -o /usr/share/munin/plugins/ipv6_ https://raw.githubusercontent.com/MorbZ/munin-ipv6/master/ipv6_
$ chmod +x /usr/share/munin/plugins/ipv6_

Setup Graphs

Total throughput:
$ ln -s /usr/share/munin/plugins/ipv6_ /etc/munin/plugins/ipv6_total

Percentages:
$ ln -s /usr/share/munin/plugins/ipv6_ /etc/munin/plugins/ipv6_percent

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