title: How to add a title to a ggplot when the title is a variable name?
body: <p>At the end of a ggplot, this works fine:</p>
  
  <pre><code>+ opts(title = expression("Chart chart_title..."))
  </code></pre>
  
  <p>But this does not:</p>
  
  <pre><code>chart_title = "foo"
  + opts(title = expression(chart_title))
  </code></pre>
  
  <p>nor this: </p>
  
  <pre><code>chart_title = "foo"
  + opts(title = chart_title)
  </code></pre>
  
  <p>How can I add a title to a ggplot when the title is a variable name?</p>