These functions are methods for the generic text.
A text label is added to the existing plot,
at the location of each point in the point pattern x,
or near the location of the midpoint of each segment
in the segment pattern x.
Additional arguments ... are passed to
text.default and may be used to control the
placement of the labels relative to the point locations, and
the size and colour of the labels.
By default, the labels are the serial numbers 1 to n, where
n is the number of points or segments in x. This can
be changed by specifying the argument labels, which should be
a vector of length n.