public function sumMetrics($label, $row)
    {
        foreach ($row as $columnName => $columnValue) {
            if (empty($columnValue)) {
                continue;
            }
            if (empty($this->data[$label][$columnName])) {
                $this->data[$label][$columnName] = 0;
            }
            if (!is_numeric($columnValue)) {
                throw new Exception("DataArray->sumMetricsPivot expects rows of numeric values, non numeric found: " . var_export($columnValue, true) . " for column $columnName");
            }
            $this->data[$label][$columnName] += $columnValue;
        }
    }