title: How using bin2d from ggplot2 distinguish two data sets?
body: <p>I have two data frames a and b</p>
  
  <pre><code>a)  
  x y z  
  1 2 a  
  3 4 a
  
  b)  
  x y z  
  2 3 b  
  4 5 b
  </code></pre>
  
  <p>After merging them with merge() I am trying to visualize them on the same graph using bin2d from ggplot2:</p>
  
  <pre><code>ggplot(data, aes(x,y))+stat_bin2d(bins=30)
  </code></pre>
  
  <p>is there a way to distinguish which region describes data set <code>a</code> and which one is from <code>b</code>?
  Maybe using different colors for each data or some contour?
  Thanks!</p>