\name{Axis}
\alias{Axis}
\title{Generic function to add an Axis to a Plot}
\description{
  Generic function to add a suitable axis to the current plot.
}
\usage{
Axis(x = NULL, at = NULL, \dots, side, labels = NULL)
}
\arguments{
  \item{x}{an object which indicates the range over which an axis should
    be drawn}
  \item{at}{the points at which tick-marks are to be drawn.}
  \item{side}{an integer specifying which side of the plot the axis is
    to be drawn on.  The axis is placed as follows: 1=below,
    2=left, 3=above and 4=right.}
  \item{labels}{this can either be a logical value specifying whether
    (numerical) annotations are to be made at the tickmarks, or a
    character or expression vector of labels to be placed at the
    tickpoints. If this is specified as a character or expression
    vector, \code{at} should be supplied and they should be the same
    length.}
  \item{\dots}{Arguments to be passed to methods and perhaps then to
    \code{\link{axis}}.}
}
\value{
  The numeric locations on the axis scale at which tick marks were drawn
  when the plot was first drawn (see Details).
  
  This function is usually invoked for its side effect, which is to add
  an axis to an already existing plot.
}
\details{
  This is a generic function.  It works in a slightly non-standard way:
  if \code{x} is supplied and non-NULL it dispatches on \code{x},
  otherwise if \code{at} is supplied and non-NULL it dispatches on \code{at},
  and the default action is to call \code{\link{axis}}, omitting argument
  \code{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 \code{\link{boxplot}}, \code{\link{contour}},
  \code{\link{coplot}}, \code{\link{filled.contour}},
  \code{\link{pairs}}, \code{\link{plot.default}}, \code{\link{rug}} and
  \code{\link{stripchart}}) will set up user coordinates and \code{Axis}
  will be called to label them appropriately.

  There are \code{"Date"}, \code{"POSIXct"} and \code{"POSIXlt"}
  methods which can pass an argument \code{format} onto the
  \link[graphics:axis.POSIXct]{appropriate} \code{axis} method.
}
\seealso{
  \code{\link{axis}}.
}
\keyword{aplot}
