code: function() {
  
              var self = this;
              var $element = $(this.element);
  
              // save this id attribute.
              self.attrId = $element.attr('id');
  
              //process the JSON data.
              // get the circle data from the data source.
              // check if the data source has different format.
              self.circleData = "data" in self.jsonData ?
                  self.jsonData.data : self.jsonData;
              // the attributes in jsonData will overwrite options.
              if('attributes' in self.jsonData) {
  
                  // the values in attributes will overwrite 
                  // the same key in options. and the result will
                  // store in options.
                  $.extend(self.options, self.jsonData.attributes);
              }
  
              // draw circles.
              self.drawCircles();
          }
docs: initialize function