This is intended for data frames with numeric columns. For more
than two columns it first calls data.matrix to convert
the data frame to a numeric matrix and then calls pairs
to produce a scatterplot matrix. This can fail and may well be
inappropriate: for example numerical conversion of dates will lose
their special meaning and a warning will be given.
For a two-column data frame it plots the second column against the
first by the most appropriate method for the first column.
For a single numeric column it uses stripchart, and for
other single-column data frames tries to find a plot method for the