Add support for tikzscale package, remove extra whitespace

[ilia-kats]

This patch adds support for the tikzscale package, making it possible to generate tikz plots with external=FALSE while still being able to scale them to the desired size. In addition, several linebreaks in the generated LaTeX code are commented out, as they led to extra whitespace being inserted before the plot, which is very annoying when trying to have multiple plots in one figure using the subfig package.

[yihui]

Thanks, but why not just insert \usepackage{tikzscale} in your LaTeX preamble? I do not quite see the point of adding this automatically to all LaTeX documents.

[ilia-kats]

You need to use \includegraphics in order for tikzscale to do its thing,
and the knitr behaviour up until now was to use \input when
external=FALSE. And if we now use \includegraphics for plain tikz files
we also need to automatically include tikzscale in the preamble.

[yihui]

I see. I think the resize.width/resize.height options were designed for rescaling \input{foo.tikz}, although this is the first time I have seen they could be useful to a real user. Here is a minimal example:

\documentclass{article}
\usepackage{tikz}
\begin{document}

<<foo, dev='tikz', external=FALSE, resize.width='.7\\textwidth'>>=
plot(1:10)
@

\end{document}

Is there a reason why you prefer external=FALSE? It makes use of your LaTeX preamble, and makes it faster to compile the document when cache=TRUE.

[ilia-kats]

The difference between resize.width and tikzscale is that resizewidth
also scales the fonts, while tikzscale keeps the fontsize constant and
just scales everything else. So if I get my graphics device size close
to what I want as final size, I can use tikzscale for finetuning using
''.7\textwidth'' or somesuch. This is also the reason why I prefer
external=FALSE, this way the fonts get rendered with the settings of the
environment the figure is in, not just the global settings.

[ilia-kats]

Oh, and as far as speed, goes, I found a workaround to make tikzexternalize work, so that's not an issue. Briefly:

\usetikzlibrary{external}
\ifx\iftikzinternalcomputation\undefined\tikzexternalize\fi

<<setup, include=FALSE, cache=FALSE>>=
    library(knitr)
    library(tikzDevice)
    options(tikzXelatexPackages = c(
    getOption( "tikzXelatexPackages" ),
    "\\newif\\iftikzinternalcomputation",
    "\\tikzinternalcomputationtrue"
    ))
@

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