Question

How to add layers in ggplot?

Answer 1

You can add multiple layers with one ggplot() and multiple geom_* functions.

See below example where I added geom_bar and a geom_point layer in the same plot.

> ggplot(mpg)+geom_bar(aes(cyl),stat = "count")+geom_point(aes(cyl,displ,col = cyl))