title: How to keep figure captions in R Markdown when fig.retina is not 1
body: <p>I'm having trouble with figure captions in html documents generated using R Markdown. If I don't specify the fig.retina option, or if I set it to 1, the output document has a figure caption. If I set it to a value that isn't 1, however, the future caption is missing but the text for it is present as the alt text for the figure. How can I keep the figure captions?</p>
  
  <p>An example:</p>
  
  <pre><code>---
  title: "Example"
  output:
    html_document:
      fig_caption: yes
      fig_retina: 2
  ---
  
  
  Text text text
  
  ```{r fig.cap="Figure 1. Some random numbers"}
  plot(rnorm(25),runif(25)) 
  ```
  </code></pre>
  
  <p>Renders to give a document with no figure caption, but if I change the value for fig.retina to 1 I get a figure caption. The same thing happens if I set fig.retina in the chunk rather than globally.</p>