Boxplot in R Programming

The box plot or boxplot in R programming is a convenient way to graphically visualizing the numerical data, group by specific data. In this article we will show you, How to Create a boxplot, Remove outlines, Format its color, adding names, adding the mean, and drawing horizontal boxplot in R Programming language with example.

Syntax of a Boxplot in R

The basic syntax to draw the Boxplot in R Programming is as shown below

boxplot(formula, data = NULL,.., subset, na.action = NULL)

and the complex syntax behind this Boxplot is:

boxplot(x, ....., range = 1.5, width = NULL, varwidth = FALSE,
     notch = FALSE, outline = TRUE, col = NULL, log = "",
     border = par("fg"), names, plot = TRUE, 
     pars = list(boxwex = 0.8, staplewex = 0.5, outwex = 0.5), 
     horizontal = FALSE, add = FALSE, at = TRUE)

There are many arguments supported by the Boxplot in R programming language, and following are some of the arguments:

data: Please specify the DataFrame, or List that contains the data to draw boxplot. In this example, it will be airquality
subset: You can restrict the barplots by specifying the vector of values. In this example, you can restrict the barplot box for August month.
x: Please specify the data from which you want to draw the boxplot. Here, you can use numeric vector, or a list containing numeric vector.
range: This argument will decide, how far the whisker extends out of box.
width: It is optional but you can use this to specify a vector that contains the widths of each box.
varwidth: It is a Boolean argument. If it is TRUE, boxes will drawn with widths proportional to the square roots of the no. of observations in the group.
border: It is an optional argument. Please specify the vector of color you want to add to the outlines of the boxplot borders.
plot: It is a Boolean argument. If it is FALSE, it will return the summaries on which the boxplots are based on.
log: You have to specify a character string of three options. If X-Axis is to be logarithmic then “x”, If Y-Axis is to be logarithmic then “y”, if both X-Axis and Y-Axis are to be logarithmic then specify either “xy” or “yx”
add: It is a Boolean argument and by default it is FALSE. If it is TRUE, boxplot should be added to an already existing plot.
horizontal: It is a Boolean argument. If it is FALSE, the boxplot will be drawn vertically. If it is TRUE, boxplot will be drawn horizontally.
at: It is a numeric vector, which gives the locations where the boxplot should be drawn. This will be very helpful, when we are adding new boxplot to existing plot region.

Before we get into the example, let us the see the data that we are going to use for this box plot example. airquality is the date set provided by the R

Return Value of a Boxplot in R Programming

In general, before we start creating a boxplot, let us see how the data is divided by the box plot. It will return the stats, outliner, group, and names. In this example we will show you, How to get the information of the same

R CODE

# R Boxplot Data 
airquality

return.value <- boxplot(airquality$Wind)
return.value

OUTPUT

Create a Boxplot in R Programming

In this example we will show you, How to create a Boxplot using the airquality data set, which is provided by the R Studio. If your requirement is to import data from external files then, I suggest you to refer R Read CSV article to understand the importing of csv file.

R CODE

# R Boxplot Data 
airquality

boxplot(airquality$Wind)

OUTPUT

ANALYSIS

airquality data set will return the output as a List. So, we are using the $ to extract the data from List.

boxplot(airquality$Wind)

Use Formula to create a Boxplot in R

In this example we will show you, How to create a Boxplot using the formula argument.

formula: It should be something like: value~group, where value is the vector of numeric values and group is the column you want to use as group by. For example, you want to draw a boxplot for countrywide sales then value = sales and group = country

R CODE

# R Boxplot Example 
airquality

boxplot(airquality$Wind~airquality$Month)

OUTPUT

Assigning names to Boxplot in R Programming

In this example we will show you, How to assign names to Boxplot, X-Axis, and Y-Axis using main, xlab, and ylab

main: You can change, or provide the Title for your Boxplot.
xlab: Please specify the label for the X-Axis
ylab: Please specify the label for the Y-Axis
las: It is used to change the Y axis values direction

R CODE

# R Boxplot Example - Changing Names
airquality

boxplot(airquality$Wind~airquality$Month,
        main = "Airquality Boxplot",
        xlab = "Months",
        ylab = "Wind",
        las = 1
        )

OUTPUT

Change Colors of a Boxplot in R

In this example we will show you, How to change the Boxplot box colors using col argument

col: Please specify the color you want to use for your Boxplot. Type colors() in your console to get the list of colors available in R programming
names: Please specify the names for the boxes. Here, we are changing the Month numbers to Month names

R CODE

# R Boxplot Example - Changing Colors, Assigning new Names
airquality

boxplot(airquality$Wind~airquality$Month,
        main = "Airquality Boxplot",
        xlab = "Months",
        ylab = "Wind",
        las = 1,
        col = c("violetred", "steelblue1", "salmon1", 
                "palegoldenrod", "olivedrab"),
        names = c("May", "June", "July", "August", "September")
        )

OUTPUT

Removing Outlines of Boxplot in R

In this example we will show you, How to remove the Outlines using outline argument.

outline: It is a Boolean argument. If it is TRUE, Boxplot will draw the outlines (that is extra dots outside the box), and if it is false all the outlines in the boxplot will be removed

R CODE

# R Boxplot Example - Removing Outlines
airquality

boxplot(airquality$Wind~airquality$Month,
        outline = FALSE,
        main = "Airquality Boxplot",
        xlab = "Months",
        ylab = "Wind",
        las = 1,
        col = c("violetred", "steelblue1", "salmon1",
                "palegoldenrod", "olivedrab"),
        names = c("May", "June", "July", 
                  "August", "September")
        )

OUTPUT

Calculating & Adding Mean to Boxplot in R

In this example we will show you, How to calculate the Mean of the each box, and how to add those mean values to existing boxplot using points function

R CODE

# R Boxplot Example - Adding Mean
airquality

boxplot(airquality$Wind~airquality$Month,
        main = "Airquality Boxplot",
        xlab = "Months",
        ylab = "Wind",
        las = 1,
        col = c("violetred", "steelblue1", "salmon1",
                "palegoldenrod", "olivedrab"),
        names = c("May", "June", "July", 
                  "August", "September")
        )

meanval <- by(airquality$Wind, airquality$Month, mean)
points(meanval, col = "white", pch = 8, cex = 1.5)

OUTPUT

ANALYSIS

Following statement will find the mean value of Wind, group by Month numbers.

meanval <- by(airquality$Wind, airquality$Month, mean)

Following statement will add that means value to the boxes. pch = 8 means star character, cex is the size of the character, and col is for color.

points(meanval, col = "white", pch = 8, cex = 1.5)

Notch argument in R Boxplot

In this example we will show you, How to draw a line on each side of the boxes using notch argument

notch: It is a Boolean argument. If it is TRUE, a notch is drawn on each side of the box. If the notches of 2 plots are overlapped then, we can say that the medians of them are same otherwise, they are different.

R CODE

# R Boxplot Example - Notch
airquality

boxplot(airquality$Wind~airquality$Month,
        notch = TRUE,
        main = "Airquality Boxplot",
        xlab = "Months",
        ylab = "Wind",
        las = 1,
        col = c("violetred", "steelblue1", "salmon1",
                "palegoldenrod", "olivedrab"),
        names = c("May", "June", "July", 
                  "August", "September")
        )

meanval <- by(airquality$Wind, airquality$Month, mean)
points(meanval, col = "white", pch = 8, cex = 1.5)

OUTPUT

Horizontal Boxplot in R Programming

In this example we will show you, How to change the default vertical boxplot into horizontal box plot using horizontal argument

R CODE

# R Boxplot Example - Horizontal Boxplot
airquality

boxplot(airquality$Wind~airquality$Month,
        notch = TRUE,
        horizontal = TRUE,
        main = "Airquality Boxplot",
        xlab = "Months",
        ylab = "Wind",
        las = 1,
        col = c("violetred", "steelblue1", "salmon1",
                "palegoldenrod", "olivedrab"),
        names = c("May", "June", "July", 
                  "August", "September")
        )

OUTPUT

Creating R Boxplot using CSV File

In this example we will show you, How to create a Boxplot using the external data. For this, we are importing data from the csv file using read.csv function. I suggest you to refer R Read CSV article to understand the steps involved in csv file import

R CODE

# R Boxplot Example - Using CSV

employee <- read.csv("Products.csv", TRUE, sep = ",", 
                     na.strings = TRUE)

boxplot(employee$SalesAmount~employee$EnglishCountryRegionName,
        main = "Products Boxplot",
        col = c("steelblue", "tomato3", "yellow2", 
                "orange4", "lawngreen", "skyblue4")
        )

Above code snippet will draw the boxplot for the Sales Amount, group by Country.

OUTPUT

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