Added plotDates parameter to XAxisLabels and:
Added plotDates parameter to XAxisLabels and:

improved year-label positioning code based on recommendation from
Lindsay Carr.
zmoore-usgs/repgen,dpattermann-usgs/repgen,dpattermann-usgs/repgen,lindsaycarr/repgen,lindsaycarr/repgen,USGS-R/repgen,USGS-R/repgen,thongsav-usgs/repgen,zmoore-usgs/repgen,lindsaycarr/repgen,USGS-R/repgen,dpattermann-usgs/repgen,mhines-usgs/repgen,thongsav-usgs/repgen,thongsav-usgs/repgen,mhines-usgs/repgen,zmoore-usgs/repgen,mhines-usgs/repgen