What are the disadvantages of a stacked bar graph?

You can only make a stacked bar graph if one of the groups is always smaller than the other.

Otherwise one bar might go right over the other one.

* * * * *

The above disadvantage does not apply to stacked bars where the bar for each variable is stacked on top of other bars. The main disadvantage is that it becomes increasingly difficult to see changes in all but the variable that is at the bottom of the stack (and so has a fixed bottom line).