PivotTable's and PivotMatrix's Field Order

PivotTable::create(array(
    ...
    'map' => array(
        'rowHeader' => function($rowHeader, $headerInfo) {
            $v = $rowHeader;
            if (isset($headerInfo['childOrder']))
                $v = $headerInfo['childOrder'] . ". " . $v;
            return $v;
        },
        'columnHeader' => function($colHeader, $headerInfo) {
            $v = $colHeader;
            if (isset($headerInfo['childOrder']))
                $v = $headerInfo['childOrder'] . ". " . $v;

            return $v;
        },
    ),
    ...
));