\Header{boxplot}{Box Plots}
\methalias{boxplot.default}{boxplot}
\methalias{boxplot.formula}{boxplot}
\keyword{hplot}{boxplot}
\begin{Description}\relax
Produce box-and-whisker plot(s) of the given (grouped) values.
\end{Description}
\begin{Usage}
\begin{verbatim}
boxplot(x, ...)

## S3 method for class 'formula':
boxplot(formula, data = NULL, ..., subset)

## Default S3 method:
boxplot(x, ..., range = 1.5, width = NULL, varwidth = FALSE,
        notch = FALSE, outline = TRUE, names, boxwex = 0.8, plot = TRUE,
        border = par("fg"), col = NULL, log = "", pars = NULL,
        horizontal = FALSE, add = FALSE, at = NULL)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{formula}] a formula, such as \code{y \textasciitilde{} x}.
\item[\code{data}] a data.frame (or list) from which the variables in
\code{formula} should be taken.
\item[\code{subset}] an optional vector specifying a subset of observations
to be used for plotting.
\item[\code{x}] for specifying data from which the boxplots are to be
produced as well as for giving graphical parameters.  Additional
unnamed  arguments specify further data, either
as separate vectors (each corresponding to a component boxplot) or
as a single list containing such vectors.  \code{\Link{NA}}s are
allowed in the data.
\item[\code{...}] For the \code{formula} method, arguments to the
default method and graphical parameters.

For the default method, unnamed arguments are additional data
vectors, and named arguments are graphical parameters in addition
to the ones given by argument \code{pars}.

\item[\code{range}] this determines how far the plot whiskers extend out
from the box.  If \code{range} is positive, the whiskers extend
to the most extreme data point which is no more than
\code{range} times the interquartile range from the box. A value
of zero causes the whiskers to extend to the data extremes.
\item[\code{width}] a vector giving the relative widths of the boxes making
up the plot.
\item[\code{varwidth}] if \code{varwidth} is \code{TRUE}, the boxes are
drawn with widths proportional to the square-roots of the number
of observations in the groups.
\item[\code{notch}] if \code{notch} is \code{TRUE}, a notch is drawn in
each side of the boxes.  If the notches of two plots do not
overlap this is \sQuote{strong evidence} that the two medians differ
(Chambers \emph{et al.}, 1983, p. 62).  See \code{\Link{boxplot.stats}}
for the calculations used.
\item[\code{outline}] if \code{outline} is not true, the boxplot lines are
not drawn.
\item[\code{names}] group labels which will be printed under each boxplot.
\item[\code{boxwex}] a scale factor to be applied to all boxes.  When there
are only a few groups, the appearance of the plot can be improved
by making the boxes narrower.
\item[\code{plot}] if \code{TRUE} (the default) then a boxplot is
produced.  If not, the summaries which the boxplots are based on
are returned.
\item[\code{border}] an optional vector of colors for the outlines of the
boxplots.  The values in \code{border} are recycled if the
length of \code{border} is less than the number of plots.
\item[\code{col}] if \code{col} is non-null it is assumed to contain colors
to be used to colour the bodies of the box plots.
\item[\code{log}] character indicating if x or y or both coordinates should
be plotted in log scale.
\item[\code{pars}] a list of graphical parameters; these are passed to
\code{\Link{bxp}} (if \code{plot} is true).
\item[\code{horizontal}] logical indicating if the boxplots should be
horizontal; default \code{FALSE} means vertical boxes.
\item[\code{add}] logical, if true \emph{add} boxplot to current plot.
\item[\code{at}] numeric vector giving the locations where the boxplots should
be drawn, particularly when \code{add = TRUE};
defaults to \code{1:n} where \code{n} is the number of boxes.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The generic function \code{boxplot} currently has a default method
(\code{boxplot.default}) and a formula interface
(\code{\Link{boxplot.formula}}).
\end{Details}
\begin{Value}
List with the following components:
\begin{ldescription}
\item[\code{stats}] a matrix, each column contains the extreme of the lower
whisker, the lower hinge, the median, the upper hinge and the
extreme of the upper whisker for one group/plot.
\item[\code{n}] a vector with the number of observations in each group.
\item[\code{conf}] a matrix where each column contains the lower and upper
extremes of the notch.
\item[\code{out}] the values of any data points which lie beyond the
extremes of the whiskers.
\item[\code{group}] a vector of the same length as \code{out} whose elements
indicate which group the outlier belongs to
\item[\code{names}] a vector of names for the groups
\end{ldescription}
\end{Value}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.

Chambers, J. M., Cleveland, W. S., Kleiner, B. and Tukey, P. A. (1983)
\emph{Graphical Methods for Data Analysis.}  Wadsworth \& Brooks/Cole.

See also \code{\Link{boxplot.stats}}.
\end{References}
\begin{SeeAlso}\relax
\code{\Link{boxplot.stats}} which does the computation,
\code{\Link{bxp}} for the plotting;
and \code{\Link{stripchart}} for an alternative (with small data
sets).
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
## boxplot on a formula:
data(InsectSprays)
boxplot(count ~ spray, data = InsectSprays, col = "lightgray")
# *add* notches (somewhat funny here):
boxplot(count ~ spray, data = InsectSprays,
        notch = TRUE, add = TRUE, col = "blue")

data(OrchardSprays)
boxplot(decrease ~ treatment, data = OrchardSprays,
        log = "y", col="bisque")

rb <- boxplot(decrease ~ treatment, data = OrchardSprays, col="bisque")
title("Comparing boxplot()s and non-robust mean +/- SD")

mn.t <- tapply(OrchardSprays$decrease, OrchardSprays$treatment, mean)
sd.t <- tapply(OrchardSprays$decrease, OrchardSprays$treatment, sd)
xi <- 0.3 + seq(rb$n)
points(xi, mn.t, col = "orange", pch = 18)
arrows(xi, mn.t - sd.t, xi, mn.t + sd.t,
       code = 3, col = "pink", angle = 75, length = .1)

## boxplot on a matrix:
mat <- cbind(Uni05 = (1:100)/21, Norm = rnorm(100),
             T5 = rt(100, df = 5), Gam2 = rgamma(100, shape = 2))
boxplot(data.frame(mat), main = "boxplot(data.frame(mat), main = ...)")
par(las=1)# all axis labels horizontal
boxplot(data.frame(mat), main = "boxplot(*, horizontal = TRUE)",
        horizontal = TRUE)

## Using 'at = ' and adding boxplots -- example idea by Roger Bivand :

data(ToothGrowth)
boxplot(len ~ dose, data = ToothGrowth,
        boxwex = 0.25, at = 1:3 - 0.2,
        subset= supp == "VC", col="yellow",
        main="Guinea Pigs' Tooth Growth",
        xlab="Vitamin C dose mg",
        ylab="tooth length", ylim=c(0,35))
boxplot(len ~ dose, data = ToothGrowth, add = TRUE,
        boxwex = 0.25, at = 1:3 + 0.2,
        subset= supp == "OJ", col="orange")
legend(2, 9, c("Ascorbic acid", "Orange juice"),
       fill = c("yellow", "orange"))
\end{ExampleCode}
\end{Examples}

