Box plot by group in R

Creating a box plot by group in base R is very straightforward. 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 specially useful if your data is in two separated 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.

boxplot(x ~ group, data = data)