ggplot: How to increase spacing between faceted plots?

Question

I have several faceted histograms (obtained with the command below) which are nicely plotted one under the other. I would like to increase the spacing between them, however,

Answer 1

Use the theme function:

library(grid)

p + theme(panel.spacing = unit(2, "lines"))

See also here: Slicing plots generated by ggplot2