Reintroduce graphcounter var to traffic_graphs.widget.php

This counter got lost in commit https://github.com/pfsense/pfsense/commit/ee965a5c7bf37b852795e1201688e3b20bf3d8d1
But the code at line 174 was still using it to offset the start time of each shown graph by 2 seconds, which I guess helps the graph data collection requests back to the pfSense to be staggered, not all having to be serviced at almost exactly the same time.
I noticed this while having a quick look at all the traffic graph widget code to see if there was an obvious place where the 2x bandwidth graphing problem happens. The "* 2" here in line 174 has nothing to do with that!
Merge pull request #1331 from phil-davis/patch-2