Box plot by group in R

Creating a box plot by group in base R is very straightforward. If you prefer ggplot2 check the box plot by group in ggplot2. In this tutorial we are going to use the following data set:

# Sample dataset
set.seed(136)
data <- data.frame(x = rnorm(300),
                   group = sample(LETTERS[1:3],
                                  size = 300,
                                  replace = TRUE))

Option 1

You can pass the variables accessing the data from the data frame using the dollar sign or subsetting the data frame. This approach is especially useful if your data is in two separate vectors.

boxplot(data$x ~ data$group)

# Equivalent to:
boxplot(data[, 1] ~ data[, 2])

# Equivalent to:
x <- data$x
y <- data$group
boxplot(x ~ y)

Option 2

Another option is passing your data frame to the data argument, so you will be able to specify only the names of the variables inside your data set. You can also add the data points to the box plot.

boxplot(x ~ group, data = data)