When reorder Fails

I am using the mpg dataset included in the ggplot2 package.

#load packages
library(tidyverse)
library(tidytext)

#view data
glimpse(mpg)

## Rows: 234
## Columns: 11
## $ manufacturer <chr> "audi", "audi", "audi", "audi", "audi", "audi", "audi", "…
## $ model        <chr> "a4", "a4", "a4", "a4", "a4", "a4", "a4", "a4 quattro", "…
## $ displ        <dbl> 1.8, 1.8, 2.0, 2.0, 2.8, 2.8, 3.1, 1.8, 1.8, 2.0, 2.0, 2.…
## $ year         <int> 1999, 1999, 2008, 2008, 1999, 1999, 2008, 1999, 1999, 200…
## $ cyl          <int> 4, 4, 4, 4, 6, 6, 6, 4, 4, 4, 4, 6, 6, 6, 6, 6, 6, 8, 8, …
## $ trans        <chr> "auto(l5)", "manual(m5)", "manual(m6)", "auto(av)", "auto…
## $ drv          <chr> "f", "f", "f", "f", "f", "f", "f", "4", "4", "4", "4", "4…
## $ cty          <int> 18, 21, 20, 21, 16, 18, 18, 18, 16, 20, 19, 15, 17, 17, 1…
## $ hwy          <int> 29, 29, 31, 30, 26, 26, 27, 26, 25, 28, 27, 25, 25, 25, 2…
## $ fl           <chr> "p", "p", "p", "p", "p", "p", "p", "p", "p", "p", "p", "p…
## $ class        <chr> "compact", "compact", "compact", "compact", "compact", "c…

mpg %>% 
  group_by(trans) %>% 
  count() %>% 
  ungroup() %>% 
  ggplot(mapping = aes(trans, n)) +
  geom_bar(stat = "identity")

This plot is ok, but how about arranging the order of trans by frequency? We can use reorder.

mpg %>% 
  group_by(trans) %>% 
  count() %>% 
  ungroup() %>% 
  ggplot(mapping = aes(reorder(trans, n), n)) +
  geom_bar(stat = "identity")

Ok, this is what we want. Let’s work with a more complicated example. We want the distribution of trans varied by drv.

mpg %>% 
  group_by(drv, trans) %>% 
  count() %>% 
  ungroup() %>% 
  ggplot(mapping = aes(reorder(trans, n), n)) +
  geom_bar(stat = "identity") +
  facet_wrap(~ drv, scale = "free")

reorder doesn’t work well with a third-dimension variable. Use reorder_within (in tidytext package) instead

mpg %>% 
  group_by(drv, trans) %>% 
  count() %>% 
  ungroup() %>% 
  ggplot(mapping = aes(reorder_within(trans, n, drv), n)) +
  geom_bar(stat = "identity") +
  facet_wrap(~ drv, scale = "free")