code: @Override
      public String toJsonString() {
          JSONArray jsonArray = new JSONArray();
          JSONArray valueArray = new JSONArray();
          valueArray.put(getData());
          jsonArray.put(valueArray);
          return jsonArray.toString();
      }
docs: /* (non-Javadoc)
  @see br.com.digilabs.jqplot.data.NumberData#toJsonString()