intermediate_ggplot2/8_label_formatting.qmd at main · fmicompbio/intermediate_ggplot2 · GitHub

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---
title: "Formatting axis labels"
author: "Charlotte Soneson, Michael Stadler"
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---

## Summary

This document provides some examples of how to format various types of labels
in `ggplot2` plots.

## Prepare data

Run the following code to load the packages and data that are used in this
document:

```{r}
#| label: prepare_data
#| code-fold: false

suppressPackageStartupMessages({
    library(dplyr)
    library(tibble)
    library(swissknife)
    library(ggplot2)
    library(ggtext)
    library(ggrepel)
})

loadExampleData("mycars")

mycars <- mycars |>
    rownames_to_column("model")

tibble(mycars)
```

## Construct basic plot

Using the `mycars` data, create the following basic plot.

```{r}
#| label: make_basic_plot

ggplot(mycars,
       aes(x = mpg, y = disp, color = cyl)) +
    geom_point(size = 4) +
    scale_color_manual(values = c(`4` = "purple", `6` = "orange",
                                  `8` = "forestgreen"),
                       name = "Number of\ncylinders") +
    labs(x = "Miles/gallon", y = "Displacement") +
    theme_bw(base_size = 13)
```

## Add labels to points

The first extension we will make to the basic plot is to add a label to each
point. We will use the `model` column. Note that the labels are added in such a
way that they do not overlap (more precisely, different labels are designed to
repel each other to avoid overlaps whenever possible).

```{r}
ggplot(mycars,
       aes(x = mpg, y = disp, color = cyl, label = model)) +
    geom_point(size = 4) +
    geom_text_repel(min.segment.length = 0, max.overlaps = Inf) +
    scale_color_manual(values = c(`4` = "purple", `6` = "orange",
                                  `8` = "forestgreen"),
                       name = "Number of\ncylinders") +
    labs(x = "Miles/gallon", y = "Displacement") +
    theme_bw(base_size = 13)
```

## Format axis labels using markdown

Next, we turn our attention to the axis labels. The `ggtext` package provides
functionality to format the axes (or other text elements) using _markdown_.
Use this functionality to add formatting (italics and bold) to the x-axis title,
and add a unit to the y-axis title (here, it is helpful to note that HTML
code can be interpreted by markdown).

```{r}
ggplot(mycars,
       aes(x = mpg, y = disp, color = cyl, label = model)) +
    geom_point(size = 4) +
    geom_text_repel(min.segment.length = 0, max.overlaps = Inf) +
    scale_color_manual(values = c(`4` = "purple", `6` = "orange",
                                  `8` = "forestgreen"),
                       name = "Number of\ncylinders") +
    labs(x = "_Miles/gallon_ (**mpg**)", y = "Displacement (in<sup>3</sup>)") +
    theme_bw(base_size = 13) +
    theme(axis.title.x.bottom = element_markdown(),
          axis.title.y.left = element_markdown())
```

## Add special characters to labels

Next, we will change the axis titles completely (the x-axis title to $\alpha$,
and the y-axis title to $\beta^4$). Note that these labels don't necessarily
make much sense in this case, but they serve as a useful example for general
special characters. We can achieve our goal in several ways - for example,
using the R `expression()` function:

```{r}
## Using expression
ggplot(mycars,
       aes(x = mpg, y = disp, color = cyl, label = model)) +
    geom_point(size = 4) +
    geom_text_repel(min.segment.length = 0, max.overlaps = Inf) +
    scale_color_manual(values = c(`4` = "purple", `6` = "orange",
                                  `8` = "forestgreen"),
                       name = "Number of\ncylinders") +
    labs(x = expression(~ alpha), y = expression(~ beta ^ 4)) +
    theme_bw(base_size = 13)
```

or by using
[Unicode characters](https://en.wikipedia.org/wiki/List_of_Unicode_characters#)
characters (note that these may not be properly displayed unless the plot
backend is set to Cairo):

```{r}
#| dev: CairoPNG

## Using Unicode
ggplot(mycars,
       aes(x = mpg, y = disp, color = cyl, label = model)) +
    geom_point(size = 4) +
    geom_text_repel(min.segment.length = 0, max.overlaps = Inf) +
    scale_color_manual(values = c(`4` = "purple", `6` = "orange",
                                  `8` = "forestgreen"),
                       name = "Number of\ncylinders") +
    labs(x = "\U03B1", y = "\U03B2\U2074") +
    theme_bw(base_size = 13)
```

## Resources

* Some
[useful examples](https://coderclub.w.uib.no/2015/05/07/expressions-in-r/) of
using `expression` and `bquote` to add special characters to plot labels.
Also see `?plotmath`.

## Session info

```{r}
#| label: session_info
sessioninfo::session_info()
```