Include same chunk twice with different paramters - r

Question

I have a long .Rnw document which consists mostly of text (typeset in LaTeX) with a few chunks here and there. I have also written a chunk which outputs a specific figure. The figure contains a plot, the values for the plot are currently read from a .csv file and some parameters like colors defined manually within the chunk.
Now I want to have the same figure in a different place in the document, but with different values for the plot and a few other parameters different. Ideally, I would like to include the chunk as a child twice, and pass parameters to it somehow, including the name of the .csv to be used for the plot values. I would hate to copy paste the chunk code with hardcoded parameters, as it is complex enough that potential changes will be difficult to synchronize.
How can I do such "parameterized reuse" of chunks?
update
As requested, a small example
This is saved as include-chunk-reuse.Rnw
<<toReuse, echo=FALSE, result='asis'>>=
l <- 25
#
\newlength{\mylength}
\setlength{\mylength}{\Sexpr{l}pt}
%Omitted: a lot of complicated LaTeX commands
\rule{\mylength}{1pt}
This is the document which is supposed to reuse the chunk. It doesn't even compile, as it complains that the same label is used twice: Error in parse_block(g[-1], g[1], params.src) : duplicate label 'toReuse'
\documentclass{article}
\begin{document}
This is some text. And now comes a 25 pt wide line.
<<first-figure, child='include-chunk-reuse.Rnw'>>=
#
This is some text. The next line is also 25 pt wide. But I would like to call the chunk in a way which makes it 50 pt wide instead.
<<second-figure, child='include-chunk-reuse.Rnw'>>=
#
\end{document}

Answer 1

For the knitr part to work simply leave out the chunk-name in the child document, then you don't have the duplicated label and the knitr part works.
Passing Parameters does not really work as far as I know, but you can just set a global variable before including the child. (For example \Sexpr{l <- 200}
You are still redefining \mylength which is why LaTeX will throw an error, so move the first definition of \mylength from the child to the main document.

Answer 2

The example below demonstrates two ways to reuse and parametrize a chunk.
Reusing Chunks
The mechanism is explained here. Basically, the simplest way to reuse a chunk is to add another empty chunk with the same label. Alternatively, the chunk option ref.label lets a chunk inherit another chunks code.
Both approaches of reusing chunks are largely equivalent – with one exception: figures generated in chunks are saved as chunklabel-i.pdf, where i is the figure index counted by chunk. Therefore, if a chunk is reused by repeating its label, figure i from the second use will overwrite figure i from the first use. This is the reason why I use ref.label (and thus distinct chunk labels) in the example below (otherwise, the points on both plots would be green).
In the example below, I used eval = FALSE in order to prevent evaluation of the masterchunk where it is defined. An alternative would be to externalize the chunk and read it by read_chunk().
Parameterizing Chunks
The two most straightforward options to "pass" parameters to a chunk are
chunk options and
global variables
Also when reusing chunks, each use can set different chunk options. The example below exploits this to set different captions.
As all chunks run in the same environment, setting a variable in an early chunk affects subsequent chunks accessing this variable. In the example below, mycolor is modified this way.
\documentclass{article}
\begin{document}
<<masterchunk, eval = FALSE>>=
plot(1:10, col = mycolor)
#
<<config1>>=
mycolor <- "red"
#
<<use1, ref.label = "masterchunk", fig.cap = "Red dots">>=
#
<<config2>>=
mycolor <- "green"
#
<<use2, ref.label = "masterchunk", fig.cap = "Green dots">>=
#
\end{document}