This demonstrates a graph of the matching of the rRegMatch type. Points are plotted in 2d; then within-group matches
are shown with dotted lines and between-group pairings with solid ones. If X has more than two columns,
the first two are used, with a warning. If Y is supplied it will be used; if
not, it will be extracted from x; if no y is found, an error is issued. Y must
have exactly two distinct values.