ggplot2 - Multiple Boxplots from Sources of Different Lengths

Question

I have a few different vectors of varying length for which I would like to generate side by side boxplots using ggplot2. This is relatively straight forward to do with the Base plotting system. However ggplot2 only takes a single data frame as input, which is difficult to create from data of varying lengths.

a <- rnorm(10)
b <- rnorm(100)
c <- rnorm(1000)
boxplot(a, b, c)

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Q: What is the correct way to draw boxplots using ggplot2 using data of varying lengths?

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Solution

ggplot uses tidy long data frames with groups (like a, b, or c) saved as separate columns. In your example, you can make a data frame with 1110 rows (10 + 100 + 1000) and two columns (the value and the group), like so:

# Make individual data frames
a <- data.frame(group = "a", value = rnorm(10))
b <- data.frame(group = "b", value = rnorm(100))
c <- data.frame(group = "c", value = rnorm(1000))

# Combine into one long data frame
plot.data <- rbind(a, b, c)

#   group      value
# 1     a  0.2322682
# 2     a -0.9681992
# ...
# 101   b  0.3422354
# 102   b  0.3495342
# ...
# 1001  c -0.6839231
# 1002  c -1.4329843

# Plot
library(ggplot2)
ggplot(plot.data, aes(x=group, y=value, fill=group)) + geom_boxplot()