Formula based column in javascript pivot grid

In this example we define a formula

    ['new_col', function (rd) {                    
        var a = rd['1998_aggr_Freight'], b = rd['1997_aggr_Freight'];
        return (isNaN(a) ? 0 : a) - (isNaN(b) ? 0 : b);
    }]

pivotCM: function (evt, ui) {
    //add new column.             
    ui.CM.push({
        title: '1998-1997',
        width: 120,
        dataIndx: 'new_col',
        dataType: 'float',
        render: function (ui) {
            //conditional background color depending upon value of cell.
            return { style: 'background-color:' + (ui.cellData > 0 ? '#aaffaa' : '#ffaaaa;') };
        },
        summary: { type: 'sum' },
        format: '$##,###.00',
        styleHead: {
            color: '#ff0000',
            'font-weight': 'bold'
        }
    })                
}