Question

How do I modify the background grid in ggplot?

Answer 1

You can use the function called background_grid(). 

This function provides a simple way to modify the background grid in ggplot2. It doesn't do anything that can't be done just the same with theme(). However, it simplifies creation of the most commonly needed variations.

background_grid
(major = c("xy", "x", "y", "only_minor", "none"),
  minor = c("xy", "x", "y", "none"), size.major = 0.2, size.minor = 0.5,
  colour.major = "grey90", colour.minor = "grey98")