By default, bar plots using geom_bar() show the count of observations for each value. We can also show other types of data, such as calculating and showing the mean instead.

Let’s say we want to see how much an animal sleeps on average by the kind of food it eats, based on the msleep dataset. The code below does just that! Recall that passing stat = "identity" to a geom_bar() layer tells ggplot2 to display values as is, rather than count the number of occurrences. We can similarly use stat = "summary", which tells ggplot2 to summarize values according to a provided function. We can specify fun = "mean" to summarize our y axis variable by calculating mean values for each value in our x axis variable.

# Filter our data to include only hours spent asleep, omitting NA values
msleep_means_df <- msleep %>%
  filter(status == "asleep") %>%
  na.omit()

# Construct a bar plot calculating and displaying means
msleep_meanbar <- 
  ggplot(msleep_means_df, 
         aes(x = diet, 
             y = hours)) + 
  labs(title="Mean Hours Asleep by Diet") +
  geom_bar(stat = "summary", 
           fun = "mean")

Here’s how this plot looks. In the msleep dataset, insectivores (animals that eat insects) sleep for fifteen hours a day on average, which is far more than animals with other diets!