Please change graph_category for all stats generated by mod_munin to "prosody" or add an option where it can be set if the default values are not ok.
In my case I have graph_category mod_muc and mod_http_upload which I would like to put under the prosody category where everything else about Prosody is located. Unfortunally setting the graph_category in munin.conf itself and overriding the defaults does not do the job. It simply has no effect. And yes, I did put it under the correct Munin host. ;-)
I'm not using Munin or mod_munin myself anymore so I'm unlikely to be motivated to work on this. Feel free to submit patches.