How to use colored text in Quarto figure captions

[friendly]

Description

In figure captions, it is often useful to say refer to colored objects in graphs by printing
the words in their colors: e.g.,
"The blue points show the data values. The regression line is shown in green, while the loess smooth is shown in red"

In the working example below, I can color words in text using an inline R expression, r colorize("red"), which works both for HTML and PDF output. However, this does not work in a figure caption. I realize that this may involve pandoc, but I'd like to find a solution that works. Can anyone help?

My working example:

---
title: "How to use colored text in Quarto figure captions"
format:
  html:
    toc: true
  pdf:
    toc: true
---
  
In figure captions, it is often useful to say refer to colored objects by printing
the words in their colors.
How can I do this with output in HTML or PDF?

Define a function to colorize text, that can be used in HTML or PDF output

```{r colorize}
# colorize text: use inline as `r colorize(text, color)` to print `text` in a given `color`
# can also be used to color a color name, as in r colorize("red")`
colorize <- function(text, color) {
  if (missing(color)) color <- text
  if (knitr::is_latex_output()) {
    sprintf("\\textcolor{%s}{%s}", color, text)
  } else if (knitr::is_html_output()) {
    sprintf("<span style='color: %s;'>%s</span>", color, text)
  } else text
}
```

I can print text in `r colorize("red")`, `r colorize("blue")`, `r colorize("green")`, ... `

But not in a figure caption.

This works without color text in the caption:

#| label: fig-test
#| out-width: "70%"
#| fig-cap: Some points should be red, some blue, some green
cols <- c("red", "blue", "green")
plot(1:10, pch=16, cex = 3, col = cols)

This doesn't work:

#| label: fig-test2
#| out-width: "70%"
#| fig-cap: Some points are `r colorize("red")`, some are `r colorize("blue")`, some are `r colorize("green")`
cols <- c("red", "blue", "green")
plot(1:10, pch=16, cex = 3, col = cols)

[friendly]

My working example can be found at: https://raw.githubusercontent.com/friendly/Vis-MLM-book/master/test/minimal-example.qmd

[mcanouil]

The inline R inside code cell is never going to work in Quarto or even in RMarkdown.

You need markdown (or use the knitr specific feature with !expr which I don't recommend especially if markdown works).

An helper could be to use the following extension: https://github.com/mcanouil/quarto-highlight-text

#| fig-cap: 'This is [Red]{colour="#b22222"}.'

The above is not tested.

The !expr would be something like:

#| fig-cap: !expr sprintf('This is %s.', colorize('red'))

[friendly]

Thanks so much @mcanouil for pointing me to !expr.
quarto-highlight-text is interesting, but seems awkward to use, at least to me.

For writing figure captions, paste() is easier than sprintf():

#| label: fig-test3
#| out-width: "70%"
#| fig-cap: !expr paste("Some points are", colorize('red'), "some are", colorize('blue'), "some are", colorize("green"))
cols <- c("red", "blue", "green")
plot(1:10, pch=16, cex = 3, col = cols)

But there is an even better solution that makes such captions easier to write, by defining global variables with color names and using them with glue::glue()

red <- colorize('red')
blue <- colorize('blue')
green <- colorize("green")
library(glue)

Then this works:

#| label: fig-test4
#| out-width: "70%"
#| fig-cap: !expr glue("Some points are ", {red}, " some are ", {blue}, "some are ", {green})
plot(1:10, pch=16, cex = 3, col = cols)

Or just with paste():

#| fig-cap: !expr paste("Some points are", red, "some are", blue, "some are", green)

[mcanouil]

My point was not to compare the various ways to concatenate strings. I simply picked one to show an example writable on a mobile.

Note that !expr is a very knitr specific feature that "works" in Quarto but not really intentionally. So you should not use it as your default strategy.

About the syntax, that's the standard Pandoc/Quarto/RMarkdowb syntax called span.
[text in red]{style="color: red;"} would work but only for HTML. My extension does this for several formats and also allows to set background colour.
It relies on nothing specific such as !expr and does not need any engines/computations. It's "native".

[friendly]

I need a solution that works for HTML and PDF, as does my colorize() function. I'll take another look at the highlight extension.

This is for a book project, where I expect to use this often, so ideally I want something that won't break and is not onerous to write.

[friendly]

One drawback of the highlight extension seems to be the need to specify colors in #rrggbb hex notation.

[mcanouil]

I would not call it a drawback especially if you intend to use colours in caption as a legend for your figure or if you are using multiple formats.

[friendly]

My complete example is now at: https://raw.githubusercontent.com/friendly/Vis-MLM-book/master/test/caption-colors.qmd

[cscheid]

This doesn't work:

#| label: fig-test2
#| out-width: "70%"
#| fig-cap: Some points are `r colorize("red")`, some are `r colorize("blue")`, some are `r colorize("green")`
cols <- c("red", "blue", "green")
plot(1:10, pch=16, cex = 3, col = cols)

Here's a general strategy. Every time you feel tempted to reach out for inline expressions and !expr in a code cell, you should consider whether the div syntax for floats could work for you. Concretely, in this case:

qmd

result

--- title: issue-10299 --- ```{r} #| echo: false #| output: false colorize <- function(text, color) { if (missing(color)) color <- text if (knitr::is_latex_output()) { sprintf("\\textcolor{%s}{%s}", color, text) } else if (knitr::is_html_output()) { sprintf("<span style='color: %s;'>%s</span>", color, text) } else text } ``` ::: {#fig-test} ```{r} #| out-width: "70%" cols <- c("red", "blue", "green") plot(1:10, pch=16, cex = 3, col = cols) ``` Some points are `r colorize("red")`, some are `r colorize("blue")`, some are `r colorize("green")`. ::: See @fig-test.