Use raster graphics

[doabell]

Currently the generated report embeds PDF figures under results/figures, like this one.
These figures are stored in a vector format, meaning the report can take minutes to load and print.

The solution is to change the graphics device to png in the chunk options on Line 127; this outputs PNGs to results/figures.
To make the images clear enough, fig.retina is also needed. Somewhere between 3 and 6 should be good for print quality.

knitr::opts_chunk$set(
  dev = "png",
  fig.retina = 4
)

Note: the $\LaTeX$ syntax for including an image is \includegraphics{image}, without file extensions.
Therefore, for the final report to use raster PNGs and not PDFs, one will need to delete the PDF figures first.

Alternatively, one can include a raw $\LaTeX$ code block inside the R markdown to specify which extensions to look for first:

```{=latex}
\DeclareGraphicsExtensions{%
    .png,.PNG,%
    .pdf,.PDF,%
    .jpg,.jpeg}
```

See also knitr: Saving graphs in both pdf and png format but using pdf files in the final document - Stack Overflow