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?

Examp

Answer 1

It looks that you want to plot them in some form of order based on the 50% value of each box plot? Taking a different dataframe as an example:

temp <- structure(list(
  Grade = c("U","G", "F", "E", "D", "C", "B", "A", "A*"), 
  n = c(20L, 13L, 4L, 13L, 36L, 94L, 28L, 50L, 27L)), 
  .Names = c("Grade", "n"), 
  class = c("tbl_df", "data.frame"), 
  row.names = c(NA, -9L))

If we plot this, we can see that the labels are messed up (A comes before A*).

library(ggplot2)
ggplot(temp) + 
geom_bar(stat="identity", aes(x=Grade, y=n))

We could order this manually as shown above, or we could decide to plot the grades in order of number of students getting each grade. This can also be done manually, but it would be better if we could automate this:

First we order the dataframe:

library(dplyr)
temp <- temp %>% arrange(n)

Then we change the levels inside the Grade column to represent the order of the data

temp$Grade <- as.vector(temp$Grade) #get rid of factors
temp$Grade = factor(temp$Grade,temp$Grade) #add ordered factors back

Running the same graph command shown above gives you data plotted with an differently ordered x axis.

Answer 2

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)