title: adding custom labels to ggplot geom_contour
body: <p>I'm trying to make a contour plot with specified breaks and labels at those breaks. I tried to add labels at the breaks using either <code>direct.label</code> or <code>geom_dl</code>, but failed.</p>
  
  <pre><code>dat &lt;- melt(volcano)
  brks &lt;- c(100, 120, 140, 160)
  g &lt;- ggplot(dat, aes(x = Var1, y = Var2, z = value)) +
    geom_contour(colour = 'black', breaks = brks)
  g
  </code></pre>
  
  <p>That part works fine, but when I try to add the labels:</p>
  
  <pre><code>direct.label(g, list("bottom.pieces", colour='black'))
  </code></pre>
  
  <p>I receive the error: <code>Need colour or fill aesthetic to infer default direct labels.</code></p>
  
  <p>And, when I try:</p>
  
  <pre><code>g + geom_dl(aes(label = brks),  method = 'bottom.pieces')
  </code></pre>
  
  <p>I get: <code>Error: Aesthetics must be either length 1 or the same as the data (5307): label, x, y, z</code></p>
  
  <p>Any suggestions?</p>