Original S code by Richard A. Becker and Allan R. Wilks.
	R version by Ray Brownrigg.
        Enhancements by Thomas P Minka and Alex Deckmyn.
Display of maps.  Projection code and larger maps are in
             separate packages ('mapproj' and 'mapdata').
mapproj (>= 1.2-0), mapdata (>= 2.3.0), sp, rnaturalearth