gglagplot: changed lag column to be lagVal

* Resolves error with geom_abline interacting with the new ggproto version of facetting introduced in ggplot2 2.2.0.
* Reversed label ordering (to maintain correct facet labels), also changed in ggplot2 2.2.0 (this may be an unintentional change in ggplot)
* Also graphical data structure is now appropriately using factors, shouldn't change the default usage, but improves making adjustments to the gglagplot output.