Stacking marks is relevant when data view includes numeric axes. That is, at least one measure has been placed on the Rows or Columns shelves. When marks are stacked, they are drawn cumulatively along an axis. When marks are not stacked, they are drawn independently along an axis. That is, they are overlapping.
Stacking marks is particularly useful for bar charts which is why Tableau automatically stacks bars. Stacking marks might be useful for other marks such as lines as well.
Control whether marks are stacked or overlapping in any given view by selecting the Analysis > Stack Marks menu item.
Either allow Tableau to automatically select whether the marks are stacked or could be specified On or Off. The default setting is Automatic. Win automatic mode, the Stack Marks menu shows whether stacked marks is on or off.
Selecting On or Off on the Stack Marks menu, switches into manual mode. The selection remains throughout any changes made in the view.