Standard of data input should be specified when stack on time/value axis

[jiawulin001]

As is stated in https://github.com/apache/echarts/issues/16700, stack on time/value axis needs user to provide elaborate dataset. It really looks like a bug when user cannot stack data correctly on 'time'/'value' axis while 'category' axis allow user to skip part of the data. This should be specified in documentation as a reminder.