How can I reorder the x axis in a plot in R?

Question

Using plot in R causes the factors on the x-axis to be alphabetically ordered.

How can I specify the order of the factors on the x-axis?

Example:

y <- 1:9
x <- c(rep("B", 3), rep("A", 3), rep("C", 3))
plot(y ~ as.factor(x))

This results in:

How can I get this to plot as "B", "A", "C"?

Answer 1

You just need to specify the levels of your factor in the order you want. So here I create a new variable x1

x1  = factor(x, levels=c("B", "C", "A"))

where

R> x1
[1] B B B A A A C C C
Levels: B C A

The plot function now works as expected.

plot(y ~ x1)