R facet labels with expressions using label_parsed

Question

I'm trying to put expressions into facet labels using label_parsed but with no success:

library(ggplot2)
mpg3 <- mpg
levels(mpg3$drv)[levels(mpg3$drv)=="4"] <- "4^{wd}"
levels(mpg3$drv)[levels(mpg3$drv)=="f"] <- "- Front %.% e^{pi * i}"
levels(mpg3$drv)[levels(mpg3$drv)=="r"] <- "4^{wd} - Front"


ggplot(mpg3, aes(x=displ, y=hwy)) + geom_point() +
  facet_grid(. ~ drv, labeller = label_parsed)

The plot that I get lacks expressions - facet labels contain the original levels of drv variable.

If I type levels(mpg3$drv) I get character(0).

Answer 1

There are two problems - firstly mpg$drv is character, not factor, and secondly, you need to set the factor labels, not the levels. I think this is what you want...

mpg3 <- mpg
mpg3$drv <- factor(mpg3$drv,
                   levels=c("4","f","r"),
                   labels=c("4^{wd}","- Front %.% e^{pi * i}","4^{wd} - Front"))

ggplot(mpg3, aes(x=displ, y=hwy)) + 
      geom_point() +
      facet_grid(. ~ drv, labeller = label_parsed)