When to use a Stacked Bar Chart
The total matters as much as the breakdown
Revenue by region split by product, headcount by department split by seniority. If your reader needs both numbers, stacking gives them both without a second chart.
You have a small number of segments
Two to five segments per bar stay readable. Beyond that the middle bands get thin, the colours get hard to tell apart, and the chart stops being useful.
You want to show composition changing across categories
Stacked bars across time periods or countries make it easy to see when one segment starts taking over — provided that segment is at the bottom of the stack.
You are comparing shares, not amounts
A 100% stacked variant normalises every bar to the same length so only the proportions differ. Use it when the totals are irrelevant or distractingly different in size.