How Quantities Combine: Understanding Data
100% stacked bars
Use 100% stacked bars to compare shares rather than amounts.
In a 100% stacked bar chart every bar is the same length, standing for 100% of its own total. Its pieces show shares, like the slices of a pie chart, but lengths are easier to compare than angles.
Shares, not amounts. Family B spends the smallest share on healthcare: 1210/7286 ≈ 16.6%, against 21.0% for A and 18.5% for C.
Electricity, 20 years apart:
- 2005: lighting 360 of 1000 kWh = 36%;
- 2025: lighting 220 of 2000 kWh = 11%, while cooling rose to 40%.
What a 100% chart cannot tell you. Fatima's garden: 50% of blooms in summer; Naveen's: 40%. That does not mean Fatima had more summer blooms: Naveen might have many more flowers altogether. A bigger share of a small total can be fewer than a smaller share of a large total.
So 100% charts compare shares within each bar, and hide the totals.
One-way street. From a stacked chart you can make a 100% chart (divide by each total). From a 100% chart you cannot get back the stacked chart: the totals are lost.
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