private function arrayToExtendedEncoding($a) {
        $xencoding = WT_GOOGLE_CHART_ENCODING;
    
        $encoding = '';
        foreach ($a as $value) {
            if ($value < 0) {
                $value = 0;
            }
            $first  = (int) ($value / 64);
            $second = $value % 64;
            $encoding .= $xencoding[(int) $first] . $xencoding[(int) $second];
        }
    
        return $encoding;
    }