title: Line plot with factor variables in R
body: <p>How can I make R draw lines between two observations according with factor variables?</p>
  
  <p>I have two 'time' points, early and late, coded as categorical</p>
  
  <pre><code>plotdata &lt;- structure(list(
                 x = structure(1:2, .Label = c("early", "late"), class = "factor"), 
                 y = 1:2
                 ),
              .Names = c("x", "y"), row.names = c(NA, -2L), class = "data.frame"
  )
  </code></pre>
  
  <p>I only get kind of a bar plot:</p>
  
  <pre><code>plot(plotdata)
  </code></pre>
  
  <p>I also tried coding the variables as 0 and 1, but then I get a continuous axis with.</p>