R: ggplot reorder x-axis with facet_wrap and coord_flip

Question

I have two data frames which I want to plot together using facet_wrap, like this:

# create data frames    
d = data.frame(
      f = rep(c("f1", "f2"), each = 4),
      x = rep(c("a", "b", "c", "d"), 2),
      y = c(0, 2, 3, 3, 2, 1, 0, 6))
    
# plot ggplot
ggplot(d, aes(x, y)) +
  geom_col() +
  facet_wrap(~ f) +
  coord_flip() 

The result:

Both plots share their x-axis, which I flipped to the side. However, the order of the x-axis follows alphabetical order. This is not what I want. Instead, I would like to order the x-axis manually using the following order from top to bottom: "a", "c", "d", "b".

I tried to pre-order the x-axis values by the following code, but this had zero effect:

d2 = d[order(c("a", "a", "c", "c", "d", "d", "b", "b")),] 

ggplot(d2, aes(x, y)) +
  geom_col() +
  facet_wrap(~ f) +
  coord_flip() 

There are a lot of other questions where people wanted to reorder the x-axes of all plots separately using different orders, such as here, but I want to do all of this at once using the same order for all plots. Does anyone have any idea how to do this while keeping it simple?

Answer 1

You need to convert your x into a factor and manually enter the values. In this case, since your axis is flipped you need to put the list in...flipped.

library(tidyverse)

d = data.frame(
  f = rep(c("f1", "f2"), each = 4),
  x = rep(c("a", "b", "c", "d"), 2),
  y = c(0, 2, 3, 3, 2, 1, 0, 6))

d$x <- factor(d$x, levels= c('b','d','c','a'))

# plot ggplot
ggplot(d, aes(x, y)) +
  geom_col() +
  facet_wrap(~ f) +
  coord_flip()