protected function fillRows(array $rows)
    {
        $row = [];
        $seriesOptions = $this->seriesOptions();
        foreach ($seriesOptions as $serieId => $serieOpts) {
            $row[$serieId] = '0';
        }

        $starts = clone($this->startDate());
        $ends = $this->endDate();
        while ($starts < $ends) {
            $x = $starts->format($this->dateFormat());
            if (!isset($rows[$x])) {
                $rows[$x] = $row;
            }
            $starts->add($this->dateInterval());
        }
        ksort($rows);
        return $rows;
    }