Hot to figure out which factor level has been mapped to which fill color on a barplot in R?

Question

I'm wandering if it's possible to get the mapping between factor levels and fill colors of bars. For example:

library(ggplot2)
library(dplyr)

d <- mtcars %>% mutate_at(vars(am, cyl, carb, gear, vs), as.factor) # make factors from vars
p <- ggplot(
  data = d,
  aes(x = am, fill = cyl)
) + geom_bar()

The result is

Now, I examine the underlying data in an object built from the plot

built <- ggplot2::ggplot_build(p)

and I can see something like this:

> built$data[[1]]
     fill  y count prop x flipped_aes PANEL group ymin ymax xmin xmax colour size linetype alpha
1 #F8766D 19     3    1 1       FALSE     1     1   16   19 0.55 1.45     NA  0.5        1    NA
2 #00BA38 16     4    1 1       FALSE     1     2   12   16 0.55 1.45     NA  0.5        1    NA
3 #619CFF 12    12    1 1       FALSE     1     3    0   12 0.55 1.45     NA  0.5        1    NA
4 #F8766D 13     8    1 2       FALSE     1     4    5   13 1.55 2.45     NA  0.5        1    NA
5 #00BA38  5     3    1 2       FALSE     1     5    2    5 1.55 2.45     NA  0.5        1    NA
6 #619CFF  2     2    1 2       FALSE     1     6    0    2 1.55 2.45     NA  0.5        1    NA

My question is, how can I get explicit information which factor level mapped to the fill aesthetic corresponds to which fill value in the above dataset? My example is very simple, but the real use-case is that all I have as the input is only a plot of class ggplot.

Answer 1

library(tidyverse)

d <- mtcars %>% mutate_at(vars(am, cyl, carb, gear, vs), as.factor) # make factors from vars

p <- ggplot(
  data = d,
  aes(x = am, fill = cyl)
) + geom_bar()

q <- ggplot_build(p)

# mappings
q[["plot"]][["mapping"]]
#> Aesthetic mapping: 
#> * `x`    -> `am`
#> * `fill` -> `cyl`

# fill levels
q[["plot"]][["scales"]][["scales"]][[2]][["palette.cache"]]
#> [1] "#F8766D" "#00BA38" "#619CFF"

# mapped aesthetic levels
q[["plot"]][["scales"]][["scales"]][[2]][["range"]][["range"]]
#> [1] "4" "6" "8"

^{Created on 2021-12-14 by the reprex package (v2.0.1)}