title: Greek letters RMarkdown PDF plots
body: <p>I'm trying to generate Greek letters in the axis labels or titles of plots in an RMarkdown file that is generating a PDF.  When I run the code the editor or in the console, I see the letter fine, but when the PDF is generated, they disappear.  In the short example below, I can see that the subscript operation is working but the Greek letter theta isn't present.</p>
  <p>I've tried changing the Typeset LaTeX in PDF using: option - I have pdfLaTeX and XeLaTex - but I see no difference.</p>
  <pre><code>---
  title: &quot;Untitled&quot;
  output: pdf_document 
  ---
  
  ```{r setup, include=FALSE}
  knitr::opts_chunk$set(echo = TRUE)
  ```
  
  ```{r}
  plot(1:100, 1:100, xlab = expression(theta[5]))
  plot(1:100, 1:100, xlab = expression('theta'[5]))
  ```
  </code></pre>
  <p><a href="https://i.stack.imgur.com/9e2aH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9e2aH.png" alt="Greek letter missing" /></a></p>
  <p><a href="https://i.stack.imgur.com/jEIO7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jEIO7.png" alt="Subscript working" /></a></p>