cellValues = function() {
        var values = [];
        visibleRows.forEach(function (row) {
          var cellValue = self.grid.getCellValue(row, self);
          var cellNumber = Number(cellValue);
          if (!isNaN(cellNumber)) {
            values.push(cellNumber);
          }
        });
        return values;
      }
@ngdoc property
@name aggregationType
@propertyOf ui.grid.class:GridOptions.columnDef
@description The aggregation that you'd like to show in the columnFooter for this
column.  Valid values are in
{@link ui.grid.service:uiGridConstants#properties_aggregationTypes uiGridConstants.aggregationTypes},
and currently include `uiGridConstants.aggregationTypes.count`,
`uiGridConstants.aggregationTypes.sum`, `uiGridConstants.aggregationTypes.avg`, `uiGridConstants.aggregationTypes.min`,
`uiGridConstants.aggregationTypes.max`.

You can also provide a function as the aggregation type, in this case your function needs to accept the full
set of visible rows, and return a value that should be shown