# Using latex2exp

#### Stefano Meschiari

#### 2022-11-27

## Usage

The `TeX` function takes a LaTeX string, parses it, and returns the closest plotmath expression suitable for use in graphics. The return value of `TeX()` can be used anywhere a plotmath expression is accepted, including plot labels, legends, and text for both base graphics and ggplot2.

Here’s a simple example:

    # Use raw strings, no need to escape backslashes.
    TeX(r"(\textbf{Euler's identity} is $e^{i\pi} + 1 = 0$.)")

In this example, `\textbf{}` is used to mark a fragment of text as bold, `$` introduces inline math mode, `^{}` typesets its contents as superscript, and `\pi` typesets the letter \(\pi\).

Starting with R 4.0, it is recommended to use the new raw string literal syntax (see `?Quotes`). The syntax looks like `r"(...)"`, where `...` can contain any character sequence, including `\`.

Another option is to escape the backslash character (`\`) for LaTeX commands, such that the command will be written as `\\command` rather than `\command`. This will also work on versions of R older than 4.0:

    # Equivalent to the previous code fragment.
    # Use regular strings, but escape the backslashes.
    TeX("\\textbf{Euler's identity} is $e^{i\\pi} + 1 = 0$.")

You can quickly preview what a translated LaTeX string would look like by using `plot`:

    plot(TeX(r'(A $\LaTeX$ formula: $\frac{2hc^2}{\lambda^5}\frac{1}{e^{\frac{hc}{\lambda k_B T}} - 1}$)'), cex=2, main="")

The following example shows plotting in base graphics:

    x <- seq(0, 4, length.out=100)
    alpha <- 1:5

    plot(x, xlim=c(0, 4), ylim=c(0, 10),
         xlab='x', ylab=TeX(r'($\alpha  x^\alpha$, where $\alpha \in \{1 \ldots 5\}$)'),
         type='n', main=TeX(r'(Using $\LaTeX$ for plotting in base graphics!)', bold=TRUE))

    for (a in alpha) {
      lines(x, a*x^a, col=a)
    }

    legend('topleft',
           legend=TeX(sprintf(r'($\alpha = %d$)', alpha)),
           lwd=1,
           col=alpha)

This example shows plotting in ggplot2:

    x <- seq(0, 4, length.out=100)
    alpha <- 1:5
    data <- map_df(alpha, ~ tibble(v=.*x^., x=x, alpha=.))

    p <- ggplot(data, aes(x=x, y=v, color=as.factor(alpha))) +
        geom_line() +
        ylab(TeX(r'($\alpha  x^\alpha$, where $\alpha \in 1\ldots 5$)')) +
        ggtitle(TeX(r'(Using $\LaTeX$ for plotting in ggplot2. I $\heartsuit$ ggplot!)')) +
        coord_cartesian(ylim=c(-1, 10)) +
        guides(color=guide_legend(title=NULL)) +
        scale_color_discrete(labels=lapply(sprintf(r'($\alpha = %d$)', alpha), TeX))
        # Note that ggplot2 legend labels must be lists of expressions, not vectors of expressions

    print(p)

Here are a few examples of what you can do with latex2exp:

    latex2exp_examples(cex=0.9)