how to get ggplot2 axis to break at variable values?

Question

This @camille code generates a nice pareto plot with ggplot.

library(tidyverse)

d <- tribble(
    ~ category, ~defect,
    "price", 80,
    "schedule", 27,
    "supplier", 66,
    "contact", 94,
    "item", 33
) %>% arrange(desc(defect)) %>%
    mutate(
        cumsum = cumsum(defect),
        freq = round(defect / sum(defect), 3),
        cum_freq = cumsum(freq)
    ) %>%
    mutate(category = as.factor(category) %>% fct_reorder(defect))

brks <- unique(d$cumsum)

ggplot(d, aes(x = fct_rev(category))) +
    geom_col(aes(y = defect)) +
    geom_point(aes(y = cumsum)) +
    geom_line(aes(y = cumsum, group = 1)) +
    scale_y_continuous(sec.axis = sec_axis(~. / max(d$cumsum), labels = scales::percent), breaks = brks)

It's almost perfect except I'd like to see the second y-axis break at the cumulative y-values. This can be achieved in base-R with the following code. But how do I do it in ggplot?

## Creating the d tribble
library(tidyverse)
d <- tribble(
  ~ category, ~defect,
  "price", 80,
  "schedule", 27,
  "supplier", 66,
  "contact", 94,
  "item", 33
)

## Creating new columns
d <- arrange(d, desc(defect)) %>%
  mutate(
    cumsum = cumsum(defect),
    freq = round(defect / sum(defect), 3),
    cum_freq = cumsum(freq)
  )

## Saving Parameters 
def_par <- par() 

## New margins
par(mar=c(5,5,4,5)) 

## bar plot, pc will hold x values for bars
pc = barplot(d$defect,  
             width = 1, space = 0.2, border = NA, axes = F,
             ylim = c(0, 1.05 * max(d$cumsum, na.rm = T)), 
             ylab = "Cummulative Counts" , cex.names = 0.7, 
             names.arg = d$category,
             main = "Pareto Chart (version 1)")

## Cumulative counts line 
lines(pc, d$cumsum, type = "b", cex = 0.7, pch = 19, col="cyan4")

## Framing plot
box(col = "grey62")

## adding axes
axis(side = 2, at = c(0, d$cumsum), las = 1, col.axis = "grey62", col = "grey62", cex.axis = 0.8)
axis(side = 4, at = c(0, d$cumsum), labels = paste(c(0, round(d$cum_freq * 100)) ,"%",sep=""), 
     las = 1, col.axis = "cyan4", col = "cyan4", cex.axis = 0.8)

## restoring default paramenter
par(def_par) 

Camille had some ideas but they still linger, "The more recent versions of ggplot2 allow for a secondary axis, but it needs to be based on a transformation of the primary axis. In this case, that means it should take the primary axis's values and divide by the maximum value to get a percentage.".

Answer 1

brks <- unique(d$cumsum)
brks2 <- unique(d$cumsum / max(d$cumsum))

ggplot(d, aes(x = fct_rev(category))) +
  geom_col(aes(y = defect)) +
  geom_point(aes(y = cumsum)) +
  geom_line(aes(y = cumsum, group = 1)) +
  scale_y_continuous(sec.axis = sec_axis(~. / max(d$cumsum), labels = scales::percent, breaks = brks2), breaks = brks)

Answer 2

The only improvement this makes over my previous code from the last question, and @Jack Brookes answer is that I eliminated the need for calculating the two sets of breaks outside of the ggplot call. Instead, I just got the breaks for the cumulative raw numbers as unique(d$cumsum) and the breaks for the cumulative frequencies as unique(d$cumfreq). On both of these, I tacked a 0 to the beginning, because otherwise there's no break placed at 0.

library(tidyverse)
library(scales)

d <- tribble(
  ~ category, ~defect,
  "price", 80,
  "schedule", 27,
  "supplier", 66,
  "contact", 94,
  "item", 33
) %>% arrange(desc(defect)) %>%
  mutate(
    cumsum = cumsum(defect),
    freq = round(defect / sum(defect), 3),
    cum_freq = cumsum(freq)
  ) %>%
  mutate(category = as.factor(category) %>% fct_reorder(defect))

ggplot(d, aes(x = fct_rev(category))) +
  geom_col(aes(y = defect)) +
  geom_point(aes(y = cumsum)) +
  geom_line(aes(y = cumsum, group = 1)) +
  scale_y_continuous(breaks = c(0, unique(d$cumsum)),
    sec.axis = sec_axis(~. / max(d$cumsum), labels = scales::percent,
       breaks = c(0, unique(d$cum_freq))) 
  ) +
  theme(panel.grid.minor = element_blank())