code: function(menuItem, e) {
          var me = this,
              menu = menuItem.parentMenu,
              hdr  = menu.activeHeader,
              view = me.view,
              store = view.store;
  
          delete me.lastGroupIndex;
          me.block();
          me.enable();
          store.group(hdr.dataIndex);
          me.pruneGroupedHeader();
          me.unblock();
          me.refreshIf();
      }
docs: Group by the header the user has clicked on.
  @private