title: set color scale for heatmap
body: <p>I am trying to compare data using heatmaps. I want to set the color scale to be the same for all of them. for example, all on the scale for values -0.5 to 0.5;  </p>
  
  <p>I have been using gplots redgreen(50). but the color intensities are rescaled on different sets of 
  data.</p>
  
  <p>for example:
  my r.matrix may range from -1 to +1; and my r1.matrix may range from -0.2 to +0.2. by plotting the two heatmap, if putting on the same color scale, I would expect the color for r1.matrix be much dimmer</p>
  
  <pre><code>hm &lt;- heatmap(r.matrix, Colv = NA, col = redgreen(50))
  hm1 &lt;- heatmap(r1.matrix, Colv = NA, col = redgreen(50))
  </code></pre>
  
  <p>The color scale will span across the data range, so with same redgreen(50) it will be same red or green for data ranging from -1 to +1 and for data ranging from -.2 to +.2. the reason to let the color span across [-1,1] on data ranging [-.2, .2] is to be able to visualize the difference in data, without looking into or knowing the data range -- an exploratory step.</p>