code: function (el) {
          if (el == null) {
              // 不指定el清空
              for (var i = 0; i < this._roots.length; i++) {
                  var root = this._roots[i];
                  if (root instanceof Group) {
                      root.delChildrenFromStorage(this);
                  }
              }
  
              this._roots = [];
              this._displayList = [];
              this._displayListLen = 0;
  
              return;
          }
  
          if (el instanceof Array) {
              for (var i = 0, l = el.length; i < l; i++) {
                  this.delRoot(el[i]);
              }
              return;
          }
  
  
          var idx = util.indexOf(this._roots, el);
          if (idx >= 0) {
              this.delFromStorage(el);
              this._roots.splice(idx, 1);
              if (el instanceof Group) {
                  el.delChildrenFromStorage(this);
              }
          }
      }
docs: 删除指定的图形(Shape)或者组(Group)
  @param {string|Array.<string>} [el] 如果为空清空整个Storage