title: dc.js use filter from other dataset
body: <p>I run into a problem when I have multiple datasets in crossfilter.
  I create one crossfilter for each dataset.
  I want to filter my data in the pie chart when I select the Year dimension in the bubble chart. They are from different datasets, but they all have Year column.</p>
  
  <p>Bubble chart:</p>
  
  <pre><code>var ndx = crossfilter(data);
  var yearDimension = ndx.dimension(function (d) { return d.FiscalYear; });
  var yearlyPerformanceGroup = yearDimension.group().reduceSum(return d.Value);
  </code></pre>
  
  <p>Pie chart</p>
  
  <pre><code>var ndxobi = crossfilter(obligation);
  var yearDimension = ndxobi.dimension(function (d) { return d.FiscalYear; });
  var obligationDimension = ndxobi.dimension(function (d) { return d.Type; });
  var valueObligationSumGroup = obligationDimension.group().reduceSum(function (d) { return d.Value; });
  </code></pre>
  
  <p>Could some one tell me how to select the year in bubble chart and then pass it to the pie chart?</p>
  
  <p>Thank you</p>