Adding subcategories to a legend in ggplot2

Question

Let's say we have this following hierarcichal data on the habitats that make up my fantasy island (which is of course always warm and sunny!)

set.seed(1)

hab_dat <- data.frame(
  habitat_type = rep(c("sea", "coast", "land"), times = 1, each = 3),
  habitat_name = c("rocky", "sandy", "seaweed",
                  "beach", "pebbles", "rockpools",
                  "fields", "hills", "forest"),
  area_km2 = sample(10:40, size =9))
  
hab_dat

I want to plot the total area of each habitat type and so write following code

hab_dat %>% 
  group_by(habitat_type) %>% 
  summarise(area_km2 = sum(area_km2)) %>%
  ggplot(aes(x = habitat_type, y = area_km2, fill = habitat_type)) +
  geom_bar(stat = "identity") +
  scale_fill_manual(values = c("gold", "forestgreen", "blue"))

Looks good, but the legend is not very informative. I would like for the habitats contained within each habitat type to be included in the legend under the appropriate habitat type, just as qualitative information. Here is an example I made in paint.

I can get a bit closer using the following code without affecting the appearance of the plot, however, I am missing the habitat_type titles and also have multiple tiles for the same colour.

hab_dat <- hab_dat %>% mutate(col = rep(c("blue", "gold", "forestgreen"), times = 1, each = 3))

pal <- setNames(as.character(hab_dat$col), as.character(hab_dat$habitat_name))

ggplot(hab_dat, aes(x = habitat_type, y = area_km2, fill = habitat_name)) +
  geom_bar(position = "stack", stat = "identity") +
  scale_fill_manual(values = pal)

I have been looking at solutions along the lines of this one but am trying for a more automated solution as my actual data is a bit larger than this, and also one that presents the colour tile once per group as per my drawing.

Answer 1

I don't think there is an elegant solution that adresses your problem. I'll suggest here that you format the labels to imply the hierarchy.

library(ggplot2)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union

set.seed(1)

hab_dat <- data.frame(
  habitat_type = rep(c("sea", "coast", "land"), times = 1, each = 3),
  habitat_name = c("rocky", "sandy", "seaweed",
                   "beach", "pebbles", "rockpools",
                   "fields", "hills", "forest"),
  area_km2 = sample(10:40, size =9))

# Format labels
labels <- split(hab_dat$habitat_name, hab_dat$habitat_type)
labels <- unlist(Map(function(top, bottom) {
  paste0(top, "\n", paste("- ", bottom, collapse = "\n"))
}, top = names(labels), bottom = labels))


hab_dat %>% 
  group_by(habitat_type) %>% 
  summarise(area_km2 = sum(area_km2)) %>%
  ggplot(aes(x = habitat_type, y = area_km2, fill = habitat_type)) +
  geom_bar(stat = "identity") +
  scale_fill_manual(
    values = c("gold", "forestgreen", "blue"),
    labels = function(i) {labels[i]} # Lookup label
  )

^{Created on 2022-07-19 by the reprex package (v2.0.1)}