Woah, can't believe anyone else knows about Arc. I was one of the unfortunate few that had to use it for a regression class in college. The language is used in the textbook Applied Regression including Computing Graphics. Since the professor also wrote the textbook and Arc has strong ties to my alma mater, I figured they just had blinders on since the rest of the statistics world was already using R.
If you are interested in trying this (awful) software out for yourself, it is available for download here http://www.stat.umn.edu/arc/software.html.