wrap_360 graph function doesn't work at all

#### Issue details
On graphing DataFlash logs, any graph that is defined as wrap_360() on mavgraphs.xml doesn't work.
It's simply skipped at graphing and not displayed.

#### Version
Tested 1.3.57, 1.3.66 and Beta
I remember it was working long time ago.

#### Platform
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

Locate the graphing path that reads mavgraphs.xml and handles the wrap_360() function when plotting DataFlash logs. Reproduce the issue with a graph defined using wrap_360(), then verify that the graph is no longer skipped and is displayed correctly.

Written by the indexing model from the issue text.