ggplot2: double axis 2

library(ggplot2)
library(dplyr)

numSamples <- 200L
DF <- data.frame(V1 = sample(1L:4L, numSamples, TRUE),
                 V2 = sample(1L:2L, numSamples, TRUE))
sum0 <- DF %>% group_by(V1) %>% summarise (mean = mean(V2), n = n())

ggplotColours <- function(n = 6, h = c(0, 360) + 15){
  if ((diff(h) %% 360) < 1) h[2] <- h[2] - 360/n
  hcl(h = (seq(h[1], h[2], length = n)), c = 100, l = 65)
}

transDblAxisForBar <- function(x, y) {
  return((y - min(y)) / (max(y) - min(y)) * extendrange(x)[2])
}

cols <- ggplotColours(2)
ggplot(sum0, aes(V1, n)) +
  geom_bar(position = "stack", stat = "identity", fill = cols[1]) +
  geom_line(aes(x = V1, y = transDblAxisForBar(n, mean)),
            col = cols[2], lwd = 1.5) +
  scale_y_continuous("n", sec.axis = sec_axis(~ . / extendrange(sum0$n)[2] *
                                                   diff(range(sum0$mean)) +
                                                   min(sum0$mean),
                                                 name = "mean"))