annotate text into x axis using ggplot2

Question

Consider the simple example:

library(ggplot2)

head(mtcars)


# create the plot
ggplot(mtcars, aes(factor(cyl))) + geom_bar() + theme_bw() +
      theme(strip.text.x = element_text(size = 20, face="bold"))+
      xlab("number of cyl") + ylab("Count")

Now we can obtain the average $mpg per cyl with:

aggregate(mpg ~ cyl, data = mtcars, FUN=mean)

How can I put these average values into the x-axis so that they appear below the corresponding cyl. Can one draw a table and somehow write that this is the ...average mpg per cyl...

Is there a specific reason to use facet_grid instead of ggtitle() to put a title on the plot? This may add unneccessary complexity to a potential answer. — Uwe
– Uwe, Commented Jun 8, 2016 at 9:07

Mike Wise · Accepted Answer · 2016-06-08 11:55:00Z

1

Here is a simple way to do it by rewriting the factor level names:

(Note that this is safe only as long as aggregate generates it table in the same order as the factor level names and without any gaps - which seems like it should be the case, but one would have to investigate to make sure. It might be safer to code it as a loop and look at the level names to make sure they match up correctly)

library(ggplot2)

head(mtcars)

adf <- aggregate(mpg ~ cyl, data = mtcars, FUN=mean)
mtcars$fcyl <- factor(mtcars$cyl)
levels(mtcars$fcyl) <- sprintf("%d \n %.1f",adf$cyl,adf$mpg)

# create the plot
ggplot(mtcars, aes(fcyl)) + geom_bar() + theme_bw() +
  theme(strip.text.x = element_text(size = 20, face="bold"))+
  xlab("number of cyl") + ylab("Count")

yielding:

edited Jun 8, 2016 at 11:55

answered Jun 8, 2016 at 11:29

Mike Wise

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1 Comment

Mike Wise Over a year ago

Thanks. Any comments?

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