Adding counts to ggmosaic, can this be done simpler?

Question

I would like to make a mosaic plot using the ggmosaic package and add the counts as shown in the example below.

The example sort of works, but I find the structure o

Answer 1

Here's a way to do this using the supplied code, but without needing to save a temporary plot. It makes use of ggplot's last_plot to access the plot object up to the most recent '+', and also accesses the data a bit more simply using layer_data, rather than ggplot_build.

library(tidyverse)
library(ggmosaic)
data <- tribble(~a, ~b, 
                1, 1, 
                1, 1, 
                1, 1, 
                1, 2, 
                2, 1,
                2, 2, 
                3, 2)

data <- data %>%
  mutate(across(c(a, b), as.factor))

ggplot(data) + 
  geom_mosaic(aes(x=product(b, a), fill=b)) +
  geom_label(data = layer_data(last_plot(), 1) %>% filter(.wt > 0),
             aes(x = (xmin + xmax) / 2,
                 y = (ymin + ymax) / 2,
                 label = .wt))

^{Created on 2020-07-05 by the reprex package (v0.3.0)}

It's still a hack, but it will save you the pain of assigning a temporary plot.