Ordering of ggplot2 facets changes after adding to a facet

Question

I changed the levels of the factor to desired ordering before using the ggplot2 's facet_wrap . It works fine as in the first example.

In the second, I added a horizontal like to one of the facets. It has changed the ordering of the facets.

How do I stop this automatic change?

mpg2 <- mpg
mpg2$drv <- factor(mpg2$drv, levels = c("r", "4", "f"))

p <- ggplot(mpg2, aes(cty, hwy)) + 
  geom_point() + 
  facet_grid(rows = vars(drv), scales = "free")
p

p + geom_hline(data = data.frame(xint = 20, drv = "r"),
          aes(yintercept = xint), linetype = "dotted", color = "blue") 

Answer 1

The dataframe for the line, column drv must have same factor levels as original dataframe mpg2:

p + geom_hline(data = data.frame(xint = 20, 
                                 drv = factor("r", levels = levels(mpg2$drv))),
               aes(yintercept = xint), linetype = "dotted", color = "blue")