How to pass a count of observations to the facet label?

Question

I would like to add a count of observations in each facet to the "title" of each facet. For instance, I have this:

library(tidyverse)    

mtcars %>% 
  ggplot(aes(x = cyl)) + geom_bar()+
  facet_wrap(~carb)

and I would like to add the frequencies from the below function to each label

table(mtcars$carb)
 1  2  3  4  6  8 
 7 10  3 10  1  1 

Thus the label for 1st row 1st column should be 1; n=7 the 1st row 2nd column 2; n=10 etc...

Answer 1

A solution using tidyverse. We can create an updated column to show the count number of carb and then a new column showing updated labels.

library(tidyverse)

mtcars %>% 
  group_by(carb) %>%
  mutate(carb_count = n()) %>%
  ungroup() %>%
  mutate(carb_updated = paste0(carb, "; n=", carb_count)) %>%
  ggplot(aes(x = cyl)) + geom_bar()+
  facet_wrap(~carb_updated)

Answer 2

I wrote this function a while back to label the facet with the standard deviation, mean or count.

#' Function will update the name with the statistic of your choice
AddNameStat <- function(df, category, count_col, stat = c("sd","mean","count"), dp= 0){

  # Create temporary data frame for analysis
  temp <- data.frame(ref = df[[category]], comp = df[[count_col]])

  # Aggregate the variables and calculate statistics
  agg_stats <- plyr::ddply(temp, "ref", summarize,
                           sd = sd(comp),
                           mean = mean(comp),
                           count = length(comp))

  # Dictionary used to replace stat name with correct symbol for plot
  labelName <- plyr::mapvalues(stat, 
                         from=c("sd","mean","count"), 
                         to=c("\u03C3", "x", "n"))

  # Updates the name based on the selected variable
  agg_stats$join <- paste0(agg_stats$ref, ": ", labelName," = ",
                           round(agg_stats[[stat]], dp))

  # Map the names
  name_map <- setNames(agg_stats$join, as.factor(agg_stats$ref))
  return(name_map[as.character(df[[category]])])
}



# Create a label for the facet
mtcars$label  <- AddNameStat(mtcars, "carb", "cyl", stat = "count")

mtcars %>% 
  ggplot(aes(x = cyl)) + geom_bar()+
  facet_wrap(~label)