A useful chart
answers a question.
Choose a visual form around the comparison someone needs to make. The most useful display is one that makes the relevant difference clear and leaves the underlying meaning intact.
Choose the comparison first.
Someone comparing categories needs a different view from someone following change over time. A bar chart can make differences in size easier to compare. A line can help reveal a sequence, provided the time intervals and meaning of the measure are clear.
Begin with a sentence describing what a reader should be able to determine. Then remove elements that do not help answer it. Decoration, excessive precision, and unnecessary chart types can compete with the information the display is meant to communicate.
Keep counts and proportions connected.
A percentage describes a part relative to a whole. It can help compare groups of different sizes, but it can also hide how many observations support a result. Show the denominator or make it easy to inspect.
In the synthetic example below, Group A and Group C both have a 75% completion share. Group A contains twice as many records. Neither view is incorrect: counts describe volume, while proportions describe the composition of each group. The reader’s question determines which view is useful.
Provide more than color.
Use clear labels, consistent ordering, and readable contrast. Do not require a reader to distinguish colors alone. A chart should have a short description and, where practical, an accessible table of values.
Interactive controls should explain what they change and work with a keyboard. Filtering should not silently remove context: describe the selected population, time period, and exclusions. A display that looks simple may still need a carefully written explanation to be understood correctly.
One dataset.
Two ways to read it.
Compare the number of completed records, or their share within each group. The view changes the question you can answer.
Group A has 36 completed records, B has 24, and C has 18. Group sizes differ.
Read the underlying values
| Group | Completed | Not completed | Total | Completion share |
|---|---|---|---|---|
| A | 36 | 12 | 48 | 75% |
| B | 24 | 24 | 48 | 50% |
| C | 18 | 6 | 24 | 75% |