Assign default id to each fig for html output

[jooyoungseo]

I am not sure whether this is a reasonable feature request, but it would be great if knitr could assign default id attribute to each fig element for html output. I think the best default would be the same as image file name (chunk label and fig.cur suffix).

There would be many possible use cases, but I am working on accessibility manipulation and accessing to id attribute is necessary.

In #2163, @yihui kindly explained how to assign id the same as options$label to out.extra via knitr hooks, however, that way cannot assign unique id value for sub-figs plotted within the same chunk.

I assume that I can employ fig.cur and fig.num options to address this, but I need more time to figure out how to make them work.

In the following code example, fig.num and fig.cur options return NULL instead of specific figure numbers.

```{r, out.extra="foo='bar' foo2='bar2'"}
hist(airquality$Ozone, col = "red")

library(tidyverse)

ggplot(mpg, aes(class)) +
  geom_bar()

knitr::opts_current$get(name = "label")
knitr::opts_current$get(name = "out.extra")
knitr::opts_current$get(name = "fig.cur")
knitr::opts_current$get(name = "fig.num")
```

Therefore, I cannot assign unique suffix to each sub-fig via knitr hooks:

knitr::opts_hooks$set(label = function(options) {
  if (!any(stringr::str_detect(options$out.extra, "id\\s?=")) || is.null(options$out.extra)) {
    number <- ifelse(is.null(options$fig.cur), 1L, options$fig.cur)

    options$out.extra <- paste(options$out.extra, paste0("id=\"", paste0(options$label, "-", number), "\""))
  }

  options
})

[yihui]

I've added a chunk option fig.id. When set to TRUE, it will generate an ID for each figure automatically. Internally, it just appends an id attribute to out.extra.

Detailed documentation at https://yihui.org/knitr/options/

[jooyoungseo]

Fantastic! This works nicely :) Thanks for the implementation, @yihui !

[jooyoungseo]

@yihui -- I was wondering if you would mind advising me how to obtain current id from inside custom knit_print(). Say, I want to redefine the knit_print() S3 function that keeps every default behavior but I want to do some tweaks with current plot id. I know the following is the nitty-gritty, but how can I pass current chunk options within the custom knit_print()?

function(options) {
  id <- sprintf("%s%s-%s", options$fig.lp, options$label, options$fig.cur)
  id <- xfun::alnum_id(id)
}

[yihui]

I'm not sure what your use case is, but I'm afraid it would be too late for knit_print() to know the plot id, because the id is not generated until when the plot is to be written to the <img> tag. At that time, knit_print() has already been called. However, if you mean to use the id from a previous chunk in a later code chunk, that would be perfectly fine (in this case, you can store the id anywhere you like, e.g., use <<- inside the fig.id function to assign the id to a global variable).

[github-actions]

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