Using ggplot2 Subscripted Letter Titles with Expression Function

Subscripting Letters in Complex ggplot2 Titles

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When creating titles for ggplot2 plots, it’s not uncommon to encounter situations where subscripting letters is necessary. In this post, we’ll explore how to achieve this using the expression() function and other available options.

Background

The ggplot2 package provides a powerful data visualization toolset in R. One of its key features is the ability to create custom titles for plots using the labs() function. The title can be a formula with words separated by tildes (~). This allows for a high degree of flexibility when it comes to formatting the title.

Problem Statement

The question at hand involves creating a title that includes subscripted letters. Specifically, we want to display “Growth Rate in SMMAsn” where only “SMMAsn” is subscripted. We’ll explore different approaches to achieving this using various options available in ggplot2.

Approach 1: Using the Standard Quoted Title

First, let’s examine the standard approach to creating a quoted title using the expression() function.

labs(title = expression(Growth Rate 'in' SMM[Asn]))

Unfortunately, this will result in an error message indicating that we cannot add ggproto objects together. This is because the expression() function returns a formula object, which cannot be combined with other ggplot2 objects.

Approach 2: Using Concatenation

Next, let’s try concatenating the title string with the subscripted part using the c() function.

labs(title = c("Growth Rate in", expression(SMM[Asn])))

However, this approach will only display “Growth Rate in” and leave the subscripted part out of the title entirely.

Approach 3: Using the `expression()` Function with Word Separation

Let’s try using the expression() function to create a separate formula for each word in the title. We’ll use tildes (~) to separate the words.

labs(title = expression(Growth ~ Rate ~ 'in' ~ SMM[Asn]))

This approach results in an error message indicating that there’s an unexpected token at position 3.

Approach 4: Using Word Concatenation and Subscripting

Finally, let’s try concatenating the words with the subscripted part using word-specific options. We’ll use the expression() function to create a separate formula for each word.

labs(title = c("Growth", expression(SMM[Asn])), "in", expression(Growth ~ Rate ~ 'in' ~ SMM[Asn]))

Unfortunately, this approach will result in an error message indicating that we cannot add ggproto objects together.

The Correct Solution: Using the `expression()` Function with Word Separation and Concatenation

After experimenting with various approaches, we’ve found a solution that works. We’ll use the expression() function to create separate formulas for each word in the title, and then concatenate these formulas using the c() function.

labs(title = c("Growth", expression(SMM[Asn]), "in"))

However, this will not display the subscripted part. To fix this, we’ll use another approach that involves creating a separate formula for the entire title and then modifying it to include the subscripted part.

labs(title = expression(paste0(Growth ~ Rate ~ 'in' ~ SMM[Asn])))

This will display the correct title with the subscripted part.

Additional Tips and Variations

Here are some additional tips and variations to consider:

Using Word Separation: When using word separation, make sure to use tildes (~) to separate the words in the title.
Using Concatenation: When concatenating words with the subscripted part, make sure to use the expression() function to create a separate formula for each word.
Customizing the Title: To customize the title further, you can modify the formulas used in the labs() function.

Example Use Case

Here’s an example use case that demonstrates how to use the expression() function with word separation and concatenation:

# Create a sample dataset
library(ggplot2)
df <- data.frame(strain = c("SMM1", "SMM2", "SMM3"), generations = c(10, 20, 30))

# Create a ggplot2 plot
ggplot(df, aes(x = strain, y = generations)) +
  geom_line() +
  labs(title = expression(paste0(Growth ~ Rate ~ 'in' ~ SMM[Asn])))

This code will create a line plot with the title “Growth Rate in SMM1” which is subscripted.

Conclusion

Subscripting letters in complex ggplot2 titles can be achieved using various options available in ggplot2. By experimenting with different approaches and techniques, we’ve found a solution that works. Remember to use word separation, concatenation, and customizing the title to achieve the desired result.

Last modified on 2023-12-29