R Notebook: do not get figure captions despite having set "fig_caption: TRUE"

Question

I add "fig_caption: TRUE" to the default R Notebook example:

---
title: "R Notebook"
output: html_notebook
fig_caption: TRUE
---
This is an [R Markdown](http://rmarkdown.rstudio.com) Notebook. When you execute code within the notebook, the results appear beneath the code. 

Try executing this chunk by clicking the *Run* button within the chunk or by placing your cursor inside it and pressing *Ctrl+Shift+Enter*. 

```{r}
plot(cars)
```

I was expecting something like a "Figure 1" caption after the plot, but do not get any. What should I do to get figure captions with an automatic numbering?