boxplot

Syntax#

boxplot(x, …) # generic function
boxplot(formula, data = NULL, …, subset, na.action = NULL) ## S3 method for class ‘formula’
boxplot(x, …, range = 1.5, width = NULL, varwidth = FALSE, notch = FALSE, outline = TRUE, names, plot = TRUE, border = par(“fg”), col = NULL, log = "", pars = list(boxwex = 0.8, staplewex = 0.5, outwex = 0.5), horizontal = FALSE, add = FALSE, at = NULL) ## Default S3 method

Parameters#

Parameters	Details (source R Documentation)
formula	a formula, such as y ~ grp, where y is a numeric vector of data values to be split into groups according to the grouping variable grp (usually a factor).
data	a data.frame (or list) from which the variables in formula should be taken.
subset	an optional vector specifying a subset of observations to be used for plotting.
na.action	a function which indicates what should happen when the data contain NAs. The default is to ignore missing values in either the response or the group.
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.
col	if col is non-null it is assumed to contain colors to be used to colour the bodies of the box plots. By default they are in the background colour.
## Create a box-and-whisker plot with boxplot() {graphics}
This example use the default `boxplot()` function and the `iris`data frame.

> head(iris)
  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5          1.4         0.2  setosa
2          4.9         3.0          1.4         0.2  setosa
3          4.7         3.2          1.3         0.2  setosa
4          4.6         3.1          1.5         0.2  setosa
5          5.0         3.6          1.4         0.2  setosa
6          5.4         3.9          1.7         0.4  setosa

Simple boxplot (Sepal.Length)

Create a box-and-whisker graph of a numerical variable

boxplot(iris[,1],xlab="Sepal.Length",ylab="Length(in centemeters)",
           main="Summary Charateristics of Sepal.Length(Iris Data)")

Boxplot of sepal length grouped by species

Create a boxplot of a numerical variable grouped by a categorical variable

boxplot(Sepal.Length~Species,data = iris)

Bring order

To change order of the box in the plot you have to change the order of the categorical variable’s levels.
For example if we want to have the order virginica - versicolor - setosa

newSpeciesOrder <- factor(iris$Species, levels=c("virginica","versicolor","setosa"))
boxplot(Sepal.Length~newSpeciesOrder,data = iris)

Change groups names

If you want to specifie a better name to your groups you can use the Names parameter. It take a vector of the size of the levels of categorical variable

boxplot(Sepal.Length~newSpeciesOrder,data = iris,names= c("name1","name2","name3"))

Small improvements

Color

col : add a vector of the size of the levels of categorical variable

boxplot(Sepal.Length~Species,data = iris,col=c("green","yellow","orange"))

Proximity of the box

boxwex: set the margin between boxes.
Left boxplot(Sepal.Length~Species,data = iris,boxwex = 0.1)
Right boxplot(Sepal.Length~Species,data = iris,boxwex = 1)

See the summaries which the boxplots are based `plot=FALSE`

To see a summary you have to put the paramater plot to FALSE.
Various results are given

> boxplot(Sepal.Length~newSpeciesOrder,data = iris,plot=FALSE)
$stats #summary of the numerical variable for the 3 groups
     [,1] [,2] [,3]
[1,]  5.6  4.9  4.3 # extreme value 
[2,]  6.2  5.6  4.8 # first quartile limit
[3,]  6.5  5.9  5.0 # median limit
[4,]  6.9  6.3  5.2 # third quartile limit
[5,]  7.9  7.0  5.8 # extreme value

$n #number of observations in each groups
[1] 50 50 50

$conf #extreme value of the notchs
         [,1]     [,2]     [,3]
[1,] 6.343588 5.743588 4.910622
[2,] 6.656412 6.056412 5.089378

$out #extreme value
[1] 4.9

$group #group in which are the extreme value
[1] 1

$names #groups names
[1] "virginica"  "versicolor" "setosa"

Additional boxplot style parameters.

Box

boxlty - box line type
boxlwd - box line width
boxcol - box line color
boxfill - box fill colors

Median

medlty - median line type (“blank” for no line)
medlwd - median line widht
medcol - median line color
medpch - median point (NA for no symbol)
medcex - median point size
medbg - median point background color

Whisker

whisklty - whisker line type
whisklwd - whisker line width
whiskcol - whisker line color

Staple

staplelty - staple line type
staplelwd - staple line width
staplecol - staple line color

Outliers

outlty - outlier line type (“blank” for no line)
outlwd - outlier line width
outcol - outlier line color
outpch - outlier point type (NA for no symbol)
outcex - outlier point size
outbg - outlier point background color

Example

Default and heavily modified plots side by side

par(mfrow=c(1,2))
# Default
boxplot(Sepal.Length ~ Species, data=iris)
# Modified
boxplot(Sepal.Length ~ Species, data=iris,
        boxlty=2, boxlwd=3, boxfill="cornflowerblue", boxcol="darkblue",
        medlty=2, medlwd=2, medcol="red", medpch=21, medcex=1, medbg="white",
        whisklty=2, whisklwd=3, whiskcol="darkblue",
        staplelty=2, staplelwd=2, staplecol="red",
        outlty=3, outlwd=3, outcol="grey", outpch=NA
        )