docs: function Plugin(element, options, jsonData) {
          // same the DOM element.
          this.element = element;
          this.jsonData = jsonData;
          // merge the options.
          // the jQuery extend function, the later object will
          // overwrite the former object.
          this.options = $.extend({}, defaultOptions, options);
          // set the plugin name.
          this._name = pluginName;
          // call the initialize function.
          this.init();
      }