[Observium] patch for rrdtool syntax

Gday folks.

I have been looking at the 'trunk' version of rrdtool where some differences exist in the parsing of 'graph' commands.

The attached patch prevents 'Unused Arguments' when rrdtool graph is called with 'empty' argument bits.

I assume I have probably 'missed' some bits, and I understand this doesn't impact Observium users yet with currently released Ubuntu rrdtools packages, but I assume at some stage it will impact, and it might be good to 'miss' that impact ?

Cheers, Peter