fct_reorder by function for only one group

Question

I have a df of public and private schools within counties, and each has an assigned value. I want to use forcats::fct_reorder to rearrange the counties by the median value, but only for the private schools. Using default forcats::fct_reorder arranges by total median, which is less useful for what I'm doing.

Reprex here:

# make df
set.seed(1)
df <-
  data.frame(
    county = rep(c("Bexar","Travis","Tarrant","Aransas"), each=20),
    type = rep(c("public","private"), each=10)
  ) %>%
  mutate(value = case_when(type == "public" ~ runif(80,0,1),
                           type == "private" ~ runif(80, 0, 10))) 
# private values are way higher than public


# relevel by median value
df %>%
  mutate(county = forcats::fct_reorder(county, value, .fun=median)) %>% 
  # this rearranges counties by total median, but I only want to arrange by median of the private schools
  
  # plot
  ggplot(aes(x=county, y = value, color = type)) +
  geom_point(position = position_dodge(
    width=.75
  )) +
  geom_boxplot(alpha=.5)

Desired output would order them by increasing median of private schools only: Aransas, Travis, Tarrant, Bexar.

thanks!

Answer 1

library(tidyverse)

set.seed(1)

df <-
  data.frame(
    county = rep(c("Bexar","Travis","Tarrant","Aransas"), each=20),
    type = rep(c("public","private"), each=10)
  ) %>%
  mutate(value = case_when(type == "public" ~ runif(80,0,1),
                           type == "private" ~ runif(80, 0, 10))) 

private_medians <-
  df %>%
  filter(type == "private") %>%
  group_by(county) %>%
  summarise(median = median(value)) %>%
  arrange(median)
private_medians
#> # A tibble: 4 x 2
#>   county  median
#>   <chr>    <dbl>
#> 1 Aransas   3.91
#> 2 Travis    4.39
#> 3 Tarrant   5.68
#> 4 Bexar     6.24

# add other counties at the end in case they do not appear in the private subset
levels <- private_medians$county %>% union(df$county %>% unique())

df %>%
  mutate(county = county %>% factor(levels = levels)) %>%
  ggplot(aes(x=county, y = value, color = type)) +
  geom_point(position = position_dodge(
    width=.75
  )) +
  geom_boxplot(alpha=.5)

^{Created on 2021-10-18 by the reprex package (v2.0.1)}