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% File src/library/graphics/man/axis.POSIXct.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2010 R Core Team
% Distributed under GPL 2 or later

\name{axis.POSIXct}
\alias{axis.POSIXct}
\alias{axis.Date}
\title{Date and Date-time Plotting Functions}
\description{
  Functions to plot objects of classes \code{"POSIXlt"},
  \code{"POSIXct"} and \code{"Date"} representing calendar dates and times. 
}
\usage{
axis.POSIXct(side, x, at, format, labels = TRUE, \dots)
axis.Date(side, x, at, format, labels = TRUE, \dots)
}
\arguments{
  \item{x, at}{A date-time or date object.}
  \item{side}{See \code{\link{axis}}.}
  \item{format}{See \code{\link{strptime}}.}
  \item{labels}{Either a logical value specifying whether
    annotations are to be made at the tickmarks, or a vector of character
    strings to be placed at the tickpoints.}
  \item{\dots}{Further arguments to be passed from or to other methods,
    typically \link{graphical parameters}.}
}
\details{
  \code{axis.POSIXct} and \code{axis.Date} work quite hard to choose
  suitable time units (years, months, days, hours, minutes or seconds)
  and a sensible output format, but this can be overridden by supplying
  a \code{format} specification.

  If \code{at} is supplied it specifies the locations of the ticks
  and labels whereas if \code{x} is specified a suitable grid of labels
  is chosen. Printing of tick labels can be suppressed by using
  \code{labels = FALSE}.
  
  The date-times for a \code{"POSIXct"} input are interpreted in the
  timezone give by the \code{"tzone"} attribute if there is one,
  otherwise the current timezone.

  The way the date-times are rendered (especially month names) is
  controlled by the locale setting of category \code{"LC_TIME"} (see
  \code{\link{Sys.setlocale}}.
}
\note{
  These functions are the workhorse for methods for \code{\link{Axis}}.
  Prior to \R 2.12.0 there were also \code{plot} methods for the
  date-time classes, but the default method has also handled those for a
  long time.
}
\value{
  The locations on the axis scale at which tick marks were drawn.
}
\seealso{
  \link{DateTimeClasses}, \link{Dates} for details of the classes.

  \code{\link{Axis}}.
}
\examples{
with(beaver1, {
time <- strptime(paste(1990, day, time \%/\% 100, time \%\% 100),
                 "\%Y \%j \%H \%M")
plot(time, temp, type = "l") # axis at 4-hour intervals.
# now label every hour on the time axis
plot(time, temp, type = "l", xaxt = "n")
r <- as.POSIXct(round(range(time), "hours"))
axis.POSIXct(1, at = seq(r[1], r[2], by = "hour"), format = "\%H")
})

plot(.leap.seconds, seq_along(.leap.seconds), type = "n", yaxt = "n",
     xlab = "leap seconds", ylab = "", bty = "n")
rug(.leap.seconds)
## or as dates
lps <- as.Date(.leap.seconds)
plot(lps, seq_along(.leap.seconds),
     type = "n", yaxt = "n", xlab = "leap seconds",
     ylab = "", bty = "n")
rug(lps)

## 100 random dates in a 10-week period
random.dates <- as.Date("2001/1/1") + 70*sort(stats::runif(100))
plot(random.dates, 1:100)
# or for a better axis labelling
plot(random.dates, 1:100, xaxt = "n")
axis.Date(1, at = seq(as.Date("2001/1/1"), max(random.dates)+6, "weeks"))
axis.Date(1, at = seq(as.Date("2001/1/1"), max(random.dates)+6, "days"),
     labels = FALSE, tcl = -0.2)
}
\keyword{utilities}
\keyword{chron}