This function can be used in an inline R expression to write out the figure
filenames without hard-coding them. For example, if you created a plot in a
code chunk with the label foo and figure path my-figure/, you
are not recommended to use hard-coded figure paths like
my-figure/foo-1.pdf (in .Rnw documents) or
 (R Markdown) in your document. Instead, you
should use \Sexpr{fig_chunk('foo', 'pdf')} or `).
You can generate plots in a code chunk but not show them inside the code
chunk by using the chunk option fig.show = 'hide'. Then you can use
this function if you want to show them elsewhere.
library(knitr)
fig_chunk("foo", "png")
fig_chunk("foo", "pdf")
fig_chunk("foo", "svg", 2) # the second plot of the chunk foofig_chunk("foo", "png", 1:5) # if the chunk foo produced 5 plots