title: Reversing a melting operation with reshape2
body: <p>Consider the following code.</p>
  
  <pre><code>library (reshape2)
  x = rnorm (20)
  y = x + rnorm (rnorm (20, sd = .01))
  
  dfr &lt;- data.frame (x, y)
  mlt &lt;- melt (dfr)
  </code></pre>
  
  <p>When I try to reverse this operation with dcast,</p>
  
  <pre><code>dcast (mlt, value ~ variable)
  </code></pre>
  
  <p>I get instead a data frame with three columns (not suitable for scatter-plotting, for instance).
  How can I reenact the original data frame with dcast?</p>