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_heap
Choose 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/