ggplot2: double axis with labels

library(ggplot2)
library(dplyr)
library(grid)
library(gridExtra)

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)
sum0 %>% ggplot(aes(x = V1, y = 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) +
  geom_text(aes(label= n), vjust = 1.5, colour = "black",
            position = position_dodge(0.9), size = 5) +
  geom_text(aes(x = V1, y = transDblAxisForBar(n, mean), label= sprintf("%.2f", mean)),
            vjust = -1.5, colour = "black", position = position_dodge(0.9), size = 5) +
  ylim(c(0, max(sum0$n) * 1.1)) +
  theme(axis.title.y = element_blank(), axis.text.y = element_blank(),
        axis.ticks.y = element_blank())