Plot annotation in R-Markdown + tikzDevice

Question

I'm preparing a presentation using R-Markdown + Beamer, and want every plot annotation to be typeset with a serif font.

Here's a minimal example (in order to run this, the code should be pasted in a .Rmd file and compiled with knitr):

---
title: An example
subtitle: which should work
output:
  beamer_presentation:
    theme: metropolis
header-includes:
  \usefonttheme[onlymath]{serif}
---

## A slide

```{r test, echo=FALSE, fig.height=3.2, fig.width=4, fig.align='center', dev='tikz', external=FALSE}
plot(2*pi*(0:19)/20, sin( 2*pi*(0:19)/20 ), xlab = '$x$', ylab='$\\sin(2\\pi x)$', pch=21, bg=rgb(0,.4,.7,.5), main='A plot using \\LaTeX', font.main=1)
```

In the generated plot, I expect the font annotations in the axes' numbering, plot title, etc, to be typeset with a serif font (the same fonts that appear in math annotations), but the actual output is typesetting with a sans-serif font. It is important that the rest of the document still typesets with sans-serif fonts.

Answer 1

That is because the plots are compiled in seperate tex files. To change anything about the plots you should change the tikzLatexPackages option. The default is

options(
tikzLatexPackages = c(
    "\\usepackage{tikz}",
    "\\usepackage[active,tightpage]{preview}",
    "\\PreviewEnvironment{pgfpicture}",
    "\\setlength\\PreviewBorder{0pt}")
),
tikzXelatexPackages = c(
    "\\usepackage{tikz}\n",
    "\\usepackage[active,tightpage,xetex]{preview}\n",
    "\\usepackage{fontspec,xunicode}\n",
    "\\PreviewEnvironment{pgfpicture}\n",
    "\\setlength\\PreviewBorder{0pt}\n"
),
tikzLualatexPackages = c(
    "\\usepackage{tikz}\n",
    "\\usepackage[active,tightpage,psfixbb]{preview}\n",
    "\\usepackage{fontspec,xunicode}\n",
    "\\PreviewEnvironment{pgfpicture}\n",
"\\setlength\\PreviewBorder{0pt}\n")
)

So adding \usefonttheme[onlymath]{serif} to that first option should do it.

You can find more info on this on page 6 of the tikzDevice manual.

Note that it is also possible to include the tex code rather then the precompiled code. Though I had some issues with this approach in the past.