title: draw lines around specific areas in geom_tile
body: <p>I have the following data frame  <code>(df1)</code> which i generated a <code>geom_tile</code> plot from it.</p>
  
  <pre><code>X Y Z
  1 1 0.343
  5 4 0.134
  10 6 0.564
  20 8 0.532
  40 9 0.235
  46 12 0.425
  </code></pre>
  
  <p>and i have another data frame which i want to use to draw the lines <code>(df2)</code>:</p>
  
  <pre><code>a b c     d
  1 1 0.05 good
  5 4 0.01 better
  10 6 0.03 middle
  20 8 0.1  bad
  40 9 0.2  bad
  46 12 0.22 bad
  </code></pre>
  
  <p>so the idea, is that <code>a</code> and <code>X</code> are the same and <code>b</code> and <code>Y</code> are the same values.</p>
  
  <p>what i want to do is to draw some lines around the <code>geom_tile</code> areas depending on the value of <code>d</code> in <code>df2</code>. so in each different area there will be a different color line e.g (good is red, better is blue, ...)</p>
  
  <p>i tried to use <code>geom_contour</code> but the problem it draws lines in a very ugly way and i wasn't able to specify coordinates in a good way ..</p>
  
  <p><em><strong>Note</em></strong></p>
  
  <ul>
  <li>some areas might not be in straight lines</li>
  <li>data frames are bigger, this is a test data to explain the idea</li>
  </ul>