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

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.

Start by reading the referenced Apache ECharts issue 16700 and the existing documentation for stacking on time, value, and category axes. Identify the appropriate documentation page from the surrounding axis and stack guidance, then specify the required data-input standard and verify that the reminder accurately distinguishes time/value axes from category axes.

Written by the indexing model from the issue text.