code: function (rel, url, $edge) {
        var that = this;
        var opts = this.options;
        $.ajax({ 'url': url, 'dataType': 'json' })
        .done(function (data) {
          if (that.$chart.data('inAjax')) {
            if (rel === 'parent') {
              if (!$.isEmptyObject(data)) {
                that.addParent($edge.parent(), data);
              }
            } else if (rel === 'children') {
              if (data.children.length) {
                that.addChildren($edge.parent(), data[rel]);
              }
            } else {
              that.addSiblings($edge.parent(), data.siblings ? data.siblings : data);
            }
          }
        })
        .fail(function () {
          console.log('Failed to get ' + rel + ' data');
        })
        .always(function () {
          that.endLoading($edge);
        });
      }
docs: load new nodes by ajax