public function getChartByIndex($index)
    {
        //check whether workshett name is set or not
        if ($this->worksheetName == '')
            throw new Exception('Worksheet name not specified');
        $strURI = Product::$baseProductUri . '/cells/' . $this->getFileName() .
            '/worksheets/' . $this->worksheetName . '/charts/' . $index;
        $signedURI = Utils::sign($strURI);
        $responseStream = Utils::processCommand($signedURI, 'GET', '', '');
        $json = json_decode($responseStream);
        return $json->Chart;

    }