public function bottom($num)
    {
        $count = $this->countData();
        $result = array();
        $start = ($count>$num)?$count-$num:0;
        for ($i=$start;$i<$count;$i++) {
            array_push($result, $this->rows[$i]);
        }
        return new DataStore($result, $this->metaData);
    }