private function get_dataset($index, $apiindex) {
        if (isset($this->data[$index])) {
            return $this->data[$index];
        }

        $data = array();
        foreach ($this->raw as $k => $v) {
            $pos = strpos($k, $apiindex);
            if ($pos !== 0) {
                continue;
            }

            $data[] = substr($k, strlen($apiindex));
        }

        $this->data[$index] = $data;

        return $data;
    }