Ext 序号递增问题

new Ext.grid.RowNumberer({header:"序号",width:35})

{header:"序号",width:35,
       renderer:function(value,metadata,record,rowIndex, colIndex, store){
             var start = store.lastOptions.params.start;
              if (isNaN(start)) 
              {
                 start = 0;
              }
              return start + rowIndex + 1;
          }
}