ggplot2. How to make geom_bar stacked chart y-range 0-100%?

Question

When using geom_bar with stat = "identity", the y-axis max is the sum total of all values. In this example, I want the y-axis max to be 100 rather than 300 and the stacked bar to show the proportion of the bar each replicate is. Does anyone know how I can do this?

dat = data.frame(sample = c(rep(1, 12),
                            rep(2, 9),
                            rep(3, 6)),
                 category = c(rep(c("A", "B", "C"), each = 4),
                              rep(c("A", "B", "C"), each = 3),
                              rep(c("A", "B", "C"), each = 2)),
                 replicate = c(rep(c("a", "b", "c", "d"), 3),
                               rep(c("a", "b", "c"), 3),
                               rep(c("a", "b"), 3)),
                 value = c(rep(25, 12),
                           rep(c(25, 25, 50), 3),
                           rep(50, 6))
                 )

ggplot(dat, 
       aes(x = sample, y = value)) +
  geom_bar(aes(fill = replicate),
           stat = "identity")

Answer 1

One way would be to pre-calculate the values before plotting.

library(dplyr)
library(ggplot2)

dat %>%
   group_by(sample) %>%
   mutate(value = value/sum(value) * 100) %>%
   ggplot() + aes(x = sample, y = value, fill = replicate) +
   geom_col()  +
   ylab('value  %')