How do you set different scale limits for different facets?

Question

Some sample data:

dfr <- data.frame(
  x = rep.int(1:10, 2),
  y = runif(20),
  g = factor(rep(letters[1:2], each = 10))
)

A simple scat

Answer 1

To contract the scale on the left plot remove the points that lie outside the range. E.g. this will reduce the y scale on the right plot to values between 0 and 0.5:

p <- ggplot(dfr, aes(x, y)) +   
     geom_point(subset=.(g == "a" | (y > 0 & y <.5))) +
     geom_blank(aes(y = ymin)) + geom_blank(aes(y = ymax)) +
     facet_wrap(~ g, scales = "free_y")

See also my answer to this question.