rstudio knitr not running pdfcrop - r

Question

I'm trying to knit the mwe below to pdf using rstudio (1.2.5019)
---
title: "Test"
author: "Me"
output:
pdf_document:
fig_caption: yes
keep_tex: yes
fig_crop: yes
---
This should be cropped
```{r}
library(knitr)
```
```{r named-chunk}
plot(x <- 0:100, dbinom(x, size=100, prob=0.6), type='l',
main="Not cropped", xlab="Looks good", ylab="Nok")
```
The output pdf looks fine except the plot has not been cropped with pdfcrop.
Here's the log output.
processing file: cropping.Rmd |..................
| 25% ordinary text without R code
|...................................
| 50% label: unnamed-chunk-1
|....................................................
| 75% ordinary text without R code
|......................................................................|
100% label: named-chunk
output file: cropping.knit.md
"C:/Program Files/RStudio/bin/pandoc/pandoc" +RTS -K512m -RTS
cropping.utf8.md --to latex --from
markdown+autolink_bare_uris+tex_math_single_backslash --output
cropping.tex --self-contained --highlight-style tango --pdf-engine
pdflatex --variable graphics --lua-filter
"C:/Users/marti/Documents/R/win-library/3.6/rmarkdown/rmd/lua/pagebreak.lua"
--lua-filter "C:/Users/marti/Documents/R/win-library/3.6/rmarkdown/rmd/lua/latex-div.lua"
--variable "geometry:margin=1in"
Output created: cropping.pdf
pdfcrop works just fine from my command line but isn't called by rstudio. It is also located by rstudio.
Sys.which('pdfcrop')
pdfcrop
"C:\texlive\2019\bin\win32\pdfcrop.exe"
What am I missing?

Answer 1

This actually solved it.
``` {r crop-hook,cache=FALSE}
knit_hooks$set(crop=hook_pdfcrop)
```
```{r named-chunk, crop=TRUE}
plot(x <- 0:100, dbinom(x, size=100, prob=0.6), type='l',
main="Not cropped", xlab="Looks good", ylab="Nok")
```
As far as I understood from the documentation this shouldn't be necessary?

Answer 2

Install pdfcrop via livetex or miktex --- the one you are using to write latex documents.