R: Box Plots

boxplot {base}

R Documentation

Box Plots

Description

Produce box-and-whisker plot(s) of the given (grouped) values.

Usage

## 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)

Arguments

x, ...

for specifying data from which the boxplots are to be produced as well as for giving graphical parameters. The named arguments in this (more precisely, in list(x, ...)) are treated as graphical parameters in addition to the ones given by argument pars. The other arguments specify the data, either as separate vectors, each corresponding to a component boxplot, or as a single list containing such vectors. NAs are allowed in the data.

range

this determines how far the plot whiskers extend out from the box. If range is positive, the whiskers extend to the most extreme data point which is no more than range times the interquartile range from the box. A value of zero causes the whiskers to extend to the data extremes.

width

a vector giving the relative widths of the boxes making up the plot.

varwidth

if varwidth is TRUE, the boxes are drawn with widths proportional to the square-roots of the number of observations in the groups.

notch

if notch is TRUE, a notch is drawn in each side of the boxes. If the notches of two plots do not overlap then the medians are significantly different at the 5 percent level.

outline

if outline is not true, the boxplot lines are not drawn.

names

group labels which will be printed under each boxplot.

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.

plot

if TRUE (the default) then a boxplot is produced. If not, the summaries which the boxplots are based on are returned.

border

an optional vector of colors for the outlines of the boxplots. The values in border are recycled if the length of border is less than the number of plots.

col

if col is non-null it is assumed to contain colors to be used to col the bodies of the box plots.

log

character indicating if x or y or both coordinates should be plotted in log scale.

pars

graphical parameters can also be passed as arguments to boxplot.

horizontal

logical indicating if the boxplots should be horizontal; default FALSE means vertical boxes.

add

logical, if true add boxplot to current plot.

at

numeric vector giving the locations where the boxplots should be drawn, particularly when add = TRUE; defaults to 1:n where n is the number of boxes.

Details

The generic function boxplot currently has a default method (boxplot.default) and a formula interface (boxplot.formula).

Value

List with the following components:

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.

n

a vector with the number of observations in each group.

conf

a matrix where each column contains the lower and upper extremes of the notch.

out

the values of any data points which lie beyond the extremes of the whiskers.

group

a vector of the same length as out whose elements indicate which group the outlier belongs to

names

a vector of names for the groups

Examples

## 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"))

[Package base version 1.5.0 ]