The following description works when using the Ubuntu "tomcat7" package:
Grab the "java/jstat__heap" plugin from munin-contrib @ github and place it into "/usr/share/munin/plugins/jstat__heap".
Link the plugin into /etc/munin/plugins
ln -s /usr/share/munin/plugins/jstat__heap /etc/munin/plugins/jstat_myname_heapChoose some useful name instead of "myname". This allows to monitor multiple JVM setups.
Configure each link you created in for example a new plugin config file named "/etc/munin/plugin-conf.d/jstat" which should contain one section per JVM looking like this
[jstat_myname_heap] user tomcat7 env.pidfilepath /var/run/tomcat7.pid env.javahome /usr/