add escape backslash to plotting files with double quotes

Created by: wilsonmr

closes #444 (closed)

simple fix, just add extra escaping backslash as suggested by @Zaharid

@enocera if you plan on running global reports tonight you might need to use this or else vp-comparefits errors at the point of doing data plots.

I tested quickly that the data plots worked for a global fit which previously was erroring for me and it seems to work fine for me, requires a recompile to replace the PLOTTING_* files for offending datasets for anyone wanting to test this