How do I access the data frame that has been passed to ggplot()?

Question

I want to set the string N=xxx as the title of my figure, where xxx is the number of observations in the data frame that I pass as the data argument to ggplot(). In my current code, I explicitly pass that data frame a second time as an argument to sprintf() which I use inside of labs():

ggplot(mtcars, aes(mpg, hp)) + 
    labs(title=sprintf("N=%i", nrow(mtcars))) + 
    geom_point()

This does produce the desired title, but it won't work with more complex tasks: I use a dplyr pipe to construct the data frame that is being plotted, and as this is a time-consuming process, I wouldn't want to repeat the pipe a second time to obtain the number of rows like in the example.

So, how do I access the data frame that has been passed as an argument to ggplot() from within the argument specifications of the functions that are used to modify the plot?

Answer 1

mtcars %>% {
  ggplot(., aes(mpg, hp)) + 
  labs(title = paste("N =", nrow(.))) + 
  geom_point()
}

Note that when wrapping the whole ggplot call in {...} curly braces, you must use the . dot pronoun for the data argument in ggplot(., ...). Then you can call back that object using the . pronoun anywhere in the call.

Answer 2

Another option that takes advantage of another of magrittr's pipe-lining features: the tee operator %T>%.

library(ggplot2)
library(magrittr)
# to solidify where the variable will be out-of-scope defined
nr <- "oops"
mtcars %T>%
  { nr <<- nrow(.) } %>%
  ggplot(aes(mpg, hp)) + 
    labs(title=sprintf("N=%i", nr)) + 
  geom_point()

(This can also be done using dplyr's do({nr <<- nrow(.)}) %>%.)

This differs from Brian's answer in two ways:

1. Subjectively "cleaner looking", in that the ggplot code is not indented within a code block. (As commented, though, the blending of different pipelines could be a negative as well.)

2. It has side-effect, by creating nr outside of the pipeline and ggplot pipes. By pre-assigning nr, I think this mitigates reaching outside of the local environment, but it's still a little sloppy.