title: dc.js heatmap colour range not getting updated after filter applied
body: <p>I'm using dc.js to create a dashboard, the dataset contains 3 types of reading from 1 year in a 24 hour basis. (in the example fiddle, i only use 15 days just for demonstration purpose)</p>
  
  <p>Here is the link to a JSFiddle to better illustrate my problem <a href="http://jsfiddle.net/table315/hLzLzhss/" rel="nofollow">http://jsfiddle.net/table315/hLzLzhss/</a></p>
  
  <p>I've used a row chart to show the total of each reading, and a heatmap to show the total reading of each day at each time.</p>
  
  <p>When 1 type of reading from the row chart has selected, the heatmap doesn't seems to recalculate the min and max of the colour domain. (a heatbox shows a light colour even the value is the highest in the current filtered dataset. )</p>
  
  <pre><code>dayAndTimeChart
    .width(30 * 24 + 500)
    .height(20 * 31)
    .dimension(dayAndTimeDimension)
    .group(dayAndTimeGroup)
    .keyAccessor(function(d) {
      return d.key[0];
    })
    .valueAccessor(function(d) {
      return d.key[1];
    })
    .colorAccessor(function(d) {
      return d.value;
    })
    .colors(["#fff7ec", "#fee8c8", "#fdd49e", "#fdbb84", "#fc8d59", "#ef6548", "#d7301f", "#b30000", "#7f0000"])
    .calculateColorDomain()
    .title(function(d) {
      return "Time:   " + d.key[0] + "\n" +
        "Day:  " + d.key[1] + "\n" +
        "value: " + d.value;
    });
  </code></pre>
  
  <p>Is there something I'm missing here? or is it a bug? if that is the case, is there any workaround?</p>