hunk options** in a setup chunk (your first R code chunk): 

--

````
```{r setup}
knitr::opts_chunk$set(fig.retina = 3)
```
````

--

While we are at it, these options are good too:

````
```{r setup}
knitr::opts_chunk$set(fig.retina = 3, warning = FALSE, message = FALSE)
```
````


--

.footnote[
[1] Read more about [images in R Markdown](http://www.zevross.com/blog/2017/06/19/tips-and-tricks-for-working-with-images-and-figures-in-r-markdown-documents)

[2] And see Hadley/Garrett's preferred [`ggplot2` ch