Create R plots from a GAMS GDX file

  Expects the GDX file to contain a 3 dimensional parameter, e.g. p(x,y)
  First index x maps to x-axis, e.g. t1,t2,...t3 (converted to numeric)
  Second index y maps to categories, e.g. oil, gas, nuclear (categories)
  Third index contains parameter values, e.g. 9 ,7 ,5 (numeric expected)
  Example:  Rscript do_r_plot.r plot4R.gdx -p p -w 50 -a TRUE