How can I plot with 2 different y-axes?

Question

I would like superimpose two scatter plots in R so that each set of points has its own (different) y-axis (i.e., in positions 2 and 4 on the figure) but the points appear su

Answer 1

One option is to make two plots side by side. ggplot2 provides a nice option for this with facet_wrap():

dat <- data.frame(x = c(rnorm(100), rnorm(100, 10, 2))
  , y = c(rnorm(100), rlnorm(100, 9, 2))
  , index = rep(1:2, each = 100)
  )

require(ggplot2)
ggplot(dat, aes(x,y)) + 
geom_point() + 
facet_wrap(~ index, scales = "free_y")