How to create high quality image output in R Notebooks?

Sample Notebook.Rmd

---
title: "R Notebook"
output: html_notebook
---

The following image uses the default chunk settings and is generally low quality.

```{r}
library(tidyverse)
ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(color = class)) +
  geom_smooth(se = FALSE) +
  labs(title = "Fuel efficiency generally decreases with engine size")

```

The following image uses chunk settings with specific figure width and height. The image is high quality, but the fonts do not scale with the figure.

```{r fig.width=12,fig.height=8}

ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(color = class)) +
  geom_smooth(se = FALSE) +
  labs(title = "Fuel efficiency generally decreases with engine size")

```

The following image uses chunk settings with a DPI of 300. The image is the same quality as the first image (low quality).

```{r dpi=300}

ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(color = class)) +
  geom_smooth(se = FALSE) +
  labs(title = "Fuel efficiency generally decreases with engine size")

```