Introduction to Custom Functions in Geom_text()

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In this article, we will explore how to use a custom-defined function to change a text label in geom_text(). We will delve into the details of the problem and provide a solution using R and the ggplot2 library.

Background on geom_text() and stat_count()

geom_text() is used to add text labels to objects in ggplot2 plots. It takes a number of arguments, including aes(), which specifies the variables that will be used for the x and y coordinates of the text.

stat_count() is a built-in function in ggplot2 that counts the occurrences of each unique value in a variable. It returns an object of class “stat_count”, which has several components, including the actual count values.

In this article, we want to use a custom-defined function to label a bar plot with variables from stat_count(). The problem arises when trying to access these variables within the geom_text() function.

Error in Finding Custom Function

When we try to use our custom function, percent_and_count(), as the value for the label argument in geom_text(), we get an error message indicating that the function cannot be found. This is because the function is not imported or loaded from a package.

The error also occurs when trying to use scales::percent() on the variable returned by stat_count(). The issue seems to stem from the way R handles functions and variables, particularly in nested environments.

Alternative Solution using utils::getAnywhere()

One possible solution to this problem is to use the utils::getAnywhere() function, which returns a list of all objects that match a given name. We can then access the object at index 1 (objs[[1]]) within this list and call it with our variables.

However, using utils::getAnywhere() seems unnecessarily complicated for what should be a simple task.

Solution Using bquote and aes_

A more elegant solution to this problem is to use the bquote() function in combination with aes_. The idea here is that when we create an expression within bquote(), R will evaluate it in the parent environment, ensuring that our custom function is found.

Here’s how you can do it:

Sample Data

First, let’s create a sample dataset:

# Sample data
set.seed(2017);
df <- data.frame(
    Type = sample(6, 100, replace = T)
);

library(ggplot2);
library(scales);

Custom Function

Next, we define our custom function:

# Your custom function
percent_and_count <- function(pct, cnt){
  paste0(percent(pct), ' (', cnt, ')')
}

Using bquote and aes_

Now, let’s create the ggplot2 object:

ggplot(df, aes(x = Type)) +
  stat_count(aes(y = ..prop..)) +
  geom_text(
    stat = "count",
    aes_(
      y = ~(..prop..), 
      label = bquote(.(percent_and_count)((..prop..), (..count..)))))

Explanation

The Role of bquote()

The key to this solution is the use of bquote() and the ~ operator. When we wrap an expression within bquote(), R will evaluate it in the parent environment, which ensures that our custom function, percent_and_count(), can be found.

By using aes_ with a quoted expression (either wrapped within bquote() or preceded by the ~ operator), we ensure that the variables are properly evaluated and that our custom function is called correctly.

The Role of aes_

In addition to using bquote(), we also use aes_ to quote expressions. This allows us to properly evaluate the variables within stat_count() and call them with our custom function.

Conclusion

Using a custom-defined function to change a text label in geom_text() can be achieved by leveraging R’s quoting system, specifically with the help of bquote() and aes_. By understanding how these functions work together, we can create more expressive and maintainable code for our ggplot2 plots.

Last modified on 2025-03-02