Axis(x = NULL, at = NULL, ..., side, labels = NULL)at should be supplied and they should be the same
    length.axis.x is supplied and non-NULL it dispatches on x,
  otherwise if at is supplied and non-NULL it dispatches on at,
  and the default action is to call axis, omitting argument
  x.  The idea is that for plots for which either or both of the axes are
  numerical but with a special interpretation, the standard plotting
  functions (including boxplot, contour,
  coplot, filled.contour,
  pairs, plot.default, rug and
  stripchart) will set up user coordinates and Axis
  will be called to label them appropriately.
  There are "Date" and "POSIXt" methods which can pass an
  argument format on to the appropriate axis method (see
  axis.POSIXct).
axis.