How to create a grouped boxplot in R?

Question

I want to merge the three datasets grouped and obtain a graph with only two boxes, 1 for A and 1 for B. Can you suggest how to get that?

I\'m tryng to create a group

Answer 1

It's easier to group them like this when data is in a long format vice wide. Starting with your vectors:

DF2 <- data.frame(
  x = c(c(A1, A2, A3), c(B1, B2, B3)),
  y = rep(c("A", "B"), each = 15),
  z = rep(rep(1:3, each=5), 2),
  stringsAsFactors = FALSE
)
str(DF2)
# 'data.frame': 30 obs. of  3 variables:
#  $ x: num  1 2 9 6 4 5 1 9 2 3 ...
#  $ y: chr  "A" "A" "A" "A" ...
#  $ z: int  1 1 1 1 1 2 2 2 2 2 ...

cols <- rainbow(3, s = 0.5)
boxplot(x ~ z + y, data = DF2,
        at = c(1:3, 5:7), col = cols,
        names = c("", "A", "", "", "B", ""), xaxs = FALSE)
legend("topleft", fill = cols, legend = c(1,2,3), horiz = T)

The use of at manually controls the placement, so the "visual grouping" is not very robust. (You can control the spacing between them with width and/or boxwex.)

You might also choose ggplot2:

library(ggplot2)
ggplot(DF2, aes(y, x, fill=factor(z))) +
  geom_boxplot()