public function format_money($field)
    {
        // use real number formatter
        $this->format_real($field);

        // negative values show in red color
        if ($this->current_row[$field] < 0) {
            $this->setTDParam($field, 'style/color', 'red');
        } else {
            $this->setTDParam($field, 'style/color', false);
        }
    }