title: crossfilter how to simply get the sum total from the values in a group
body: <p>Given a crossfilter group that reduce to a key-value pair of something like:</p>
  
  <pre><code>[
     { key: A, value: 75 },
     { key: B, value: 35 }
  ]
  </code></pre>
  
  <p>(pie chart type data), how to you go about getting the sum of the values (110 in this case)? is there a quick xfilter method I am unable to find?</p>
  
  <p><a href="https://jsfiddle.net/knycsLLy/1/" rel="nofollow noreferrer">Example Fiddle Here</a></p>