pdfcrop x x
is executed on a PDF plot file, and
convert x -trim x
is executed for other types of plot files, where
x
is the plot filename.
plot_crop(x, quiet = !opts_knit$get("progress"))
pdfcrop
is often shipped with a LaTeX distribution, and
convert
is a command in ImageMagick (Windows users may have to put
the bin path of ImageMagick into the PATH variable).
convert
command in ImageMagick:
http://www.imagemagick.org/script/convert.php