public static String getData() {

        String data = "";
        for ( int i = 0; i < histogramDatas.size(); i ++ ) {
            HistogramData histogramData = histogramDatas.get(i);
            data += ",{y:" + histogramData.data_y + ",color:colors[" + i +"],drilldown:{ name: '"
                    + histogramData.categories + "', categories: [";

            String keyArry = "";
            for ( String key:histogramData.drilldown.keySet() ) {
                keyArry += ",'" + key + "'";
            }
            data += keyArry.substring(1) + "],data:[";

            String dataArry = "";
            for ( String key:histogramData.drilldown.keySet() ) {
                String value = histogramData.drilldown.get(key);
                dataArry += "," + value;
            }
            data += dataArry.substring(1) + "],color: colors[" + i + "]}}";
        }
        data = data.substring(1);
        return data;
    }