Recommend pdf fig.ext for tikz

[jsta]

I have found that specifying a png fig.ext creates poor quality pixelations of the final tikz output.

[yihui]

Thanks! I used png mostly because it can be viewed directly on Github: https://github.com/yihui/knitr-examples/blob/master/058-engine-tikz.md Of course, PDF will be of higher quality.