title: In R and ggvis, how to use LaTeX-typeset expressions in axes labels?
body: <p>In R, when drawing a plot using <code>ggvis</code>, how to use LaTeX-typeset expressions in axes labels?</p>
  
  <pre><code>add_axis("x", title="$x_n$")
  </code></pre>
  
  <p>does not work.</p>