Number formatting axis labels in ggplot2?

One needs to load library(scales) before attempting this.

More generally, you can control some nice parameters using the "scales" package. One of its functions is number_format().

library(ggplot2)
library(scales)
p <- ggplot(mpg, aes(displ, cty)) + geom_point()

For formatting your numbers, you can use the function number_format(). It offers some nice possibilities, such as controlling the number of decimals (here 2 decimals) and your decimal mark (here ',' instead of '.')

p + scale_y_continuous(
  labels = scales::number_format(accuracy = 0.01,
                                 decimal.mark = ','))