[Observium] Feature idea: custom graphs

Prior to the recent changes to Custom OIDs (r8738~r8741) we had been abusing the feature to create custom graphs containing multiple hosts. For example we had a graph of emails delivered that combined all of our email servers in one. Now with OID discovery added and manual association removed that graph goes from 20 hosts to 600 and is no longer very useful.

I think moving some of the previous functionality of Custom OIDs into it's own Custom Graph feature would be very useful and welcome feedback on the idea.

Thanks, Jesse DeFer Arizona State University