Add the percentage of different types of observations in a geom_count plot in ggplot2

A MWE would be:

df <- tibble(ID = c(1:12), 
             Var1 = c(A, B, C, C, C, A, B, B, C, A, B, C),
             Var2 = c(B, B, A, A, C, C, A, A, B, C, A, C)
)

# draw the following plot showing the frequency of each type of observation
# with dot size.
ggplot(df, aes(x = factor(Var1, level=c('A', 'B', 'C')), 
                                  y = factor(Var2, level=c('A', 'B', 'C')) )) + 
   geom_count(show.legend = F) + 
   labs(x = "Var1", y = "Var2") + 
   theme_bw()

The resulted figure looks like this:

What I want to do is to add a number beside each dot in the figure showing the percentage of the observation of that dot.

I can calculate the percentage and mannually add annotations for each dot. However, I have five treatments like this, and each treatment has 9 percentage numbers to add in the plot. Adding them one by one mannually would be very tedious. Is there an easy way out? I believe there is one since R is very powerful!

Thanks a lot for all your help.

Solution

Using a geom_text with stat="sum" and after_stat() you can add a label of the proportions like so where n is the name of the column containing the counts computed by stat="sum" under the hood:

set.seed(123)

df <- data.frame(
  ID = c(1:12),
  Var1 = sample(LETTERS[1:3], 12, replace = TRUE),
  Var2 = sample(LETTERS[1:3], 12, replace = TRUE)
)

library(ggplot2)

ggplot(df, aes(
  x = factor(Var1, level = c("A", "B", "C")),
  y = factor(Var2, level = c("A", "B", "C"))
)) +
  geom_count(show.legend = FALSE) +
  geom_text(
    aes(
      label = scales::percent(after_stat(n / sum(n)))
    ),
    size = 10 / .pt,
    stat = "sum",
    vjust = 0,
    nudge_y = .2
  ) +
  labs(x = "Var1", y = "Var2") +
  theme_bw()