title: How to obtain custom aggregate totals in UI Grid's footer
body: <p>Is there a way I could obtain a custom total in UI-Grid?</p>
  
  <p>For example I have the following <a href="http://plnkr.co/edit/0kRRvyBkyC85MbxDceHT?p=preview" rel="nofollow noreferrer">Plunkr</a></p>
  
  <p>I would like to only add up rows with names Terry Clay and Nieves Mack and display that as a total. I.E: <strong>I would like a total of the sum of a specific number of rows</strong>. Is it doable? I've been looking for a way to do it and got to <code>customTreeAggregationFn</code> and <code>customTreeAggregationFinalizerFn</code> as shown in the plunkr. However I'm unsure as to how to access the values in those specific rows to add them up and display as part of the footer.</p>
  
  <p>Here's the code in the Plunkr</p>
  
  <pre><code>$scope.gridOptions = {
      showGridFooter: true,
      showColumnFooter: true,
      enableFiltering: true,
      columnDefs: [
          { field: 'name', width: '13%' },
          { field: 'address.street',aggregationType: uiGridConstants.aggregationTypes.sum, width: '13%' },
          { field: 'age', aggregationType: uiGridConstants.aggregationTypes.avg, aggregationHideLabel: true, width: '13%' },
          { name: 'ageMin', field: 'age', aggregationType: uiGridConstants.aggregationTypes.min, width: '13%', displayName: 'Age for min' },
          { name: 'ageMax', field: 'age', aggregationType: uiGridConstants.aggregationTypes.max, width: '13%', displayName: 'Age for max' },
          { name: 'customCellTemplate', field: 'age', width: '14%', 
            customTreeAggregationFn : function( aggregation, fieldValue, numValue, row ) {
            if(row.entity.name.value === 'Terry Clay' || row.entity.name.value === 'Nieves Mack'){
             aggregation.value += row.entity.age;
            }
            },
            customTreeAggregationFinalizerFn: function( aggregation ) {
             aggregation.rendered = aggregation.value;
            }
          },
          { name: 'registered', field: 'registered', width: '20%', cellFilter: 'date', footerCellFilter: 'date', aggregationType: uiGridConstants.aggregationTypes.max }
      ],
      data: data,
      onRegisterApi: function(gridApi) {
              $scope.gridApi = gridApi;
      }
  };
  </code></pre>
  
  <p>Please help, I've been having a bunch of problems with UI Grid and it's lack of support, so hopefully that's not the case here.</p>