title: use of mathematical annotation as factor in R
body: <p>I refer to my <a href="https://stackoverflow.com/questions/7508754/showing-utf-8-character-in-chart-generated-by-ggplot2">previous question</a>, and want to know more about characteristics of <code>factor</code> in R.</p>
  
  <p>Let say I have a dataset like this:</p>
  
  <pre><code>temp &lt;- data.frame(x=letters[1:5],
                     y=1:5)
  plot(temp)
  </code></pre>
  
  <p>I can change the label of <code>x</code> easily to another character:</p>
  
  <pre><code>levels(temp[,"x"]) &lt;- letters[6:10]
  </code></pre>
  
  <p>But if I want to change it into some <code>expression</code></p>
  
  <pre><code>levels(temp[,"x"]) &lt;- c(expression(x&gt;=1),
                          expression(x&gt;=2),
                          expression(x&gt;=3),
                          expression(x&gt;=4),
                          expression(x&gt;=5))
  </code></pre>
  
  <p>The <code>&gt;=</code> sign will  not change accordingly in the plot. And I found that <code>class(levels(temp[,"x"]))</code> is <code>character</code>, but <code>expression(x&gt;=1)</code> is not.</p>
  
  <p>If I want to add some mathematical annotation as factor, what can I do?</p>