Set Tufte figure caption inside knitr chunk

Question

I am using the new Tufte package from Rstudio to generate a report. I would like to have a fairly complex margin note next to my figure. It is quite cumbersome to build the caption in the chunk header, so I want to set the fig.cap option inside the chunk.

I have tried this as a test case, but it doesn't seem to work.

knitr::opts_current$set("fig.cap"="The Caption")

does anyone know how to get this done properly?

A related question about inline captions was pointed out. While the concepts are similar, That question was very specific to the latex aspects of knitr. Even after having figured out the answer to my issue, reading that question and answer was still too far afield to have been an answer to my question in my opinion.

Answer 1

I finally cracked it and will leave the answer to hopefully save some others a few hours of research time.

knitr::opts_current$set

doesn't do anything. you can use

knitr::opts_current$get

to read the current chunk settings. The settings for the current chunk can only be set in the chunk header.

To solve my problem of not wanting to insert a complex caption into the header I used the following pieces:

1. Set the package option to evaluate the caption after the figure is created by using this:

   knitr::opts_knit$set(eval.after = 'fig.cap')
   

2. In the chunk header, include something like this:

   fig.cap=varName
   

3. Inside the chunk, set varName to the the value you want for the figure caption.

   varName <- paste("This is a sample caption evaluated on", Sys.Date(), sep=" ")
   

I haven't played around with multiple figures and captions within the same chunk yet, so I am not sure how that might work, but this definitely works for a single figure chunk.