This question was posted about 3 years ago and the answer was to use a
third party tool. That seems a bit ridiculous when the functionality
seems to be inherently in Excel.

The problem: I have a stacked bar chart in Excel and would like to
show the percentages for each of the datasets in the bars. When I go
to Format Format Data Series Data Labels, I can select Series
Name, Category Name, and Value for "Label Contains" but Percentage and
Bubble Size are both unavailable (greyed out). I need the percentages.
How do I get those to display?