the rrdGraphCtrl is only concerned with the controlling the rrdGraphPng objects, so it would not make sense to specify the name of an rrd file there.
it would not make sense todo it in the properties of rrdGraphPng either, because rrdGraphPng can actually concern multiple img tags.
the right place to specify the name of the rrd file is in the data-src-template as just another argument to the get call ... no involvement with the javascript side at all ...
on the server side you would then have to accept the name of the rrd file as a parameter
Don't know if the patch in rrdGraphPng.js is appropriate.