03 / Visualization
Choose the question first
A clear chart starts with the comparison a reader needs to make. Decoration comes later, if it is needed at all.
Match the shape to the task
| Question | Useful starting point |
|---|---|
| Which category is larger? | Sorted horizontal bars |
| How did a value change over time? | Line chart, or bars for a few periods |
| How are values distributed? | Histogram with explicit bin widths |
| Do two measurements move together? | Scatterplot with context |
These are starting points rather than rules. A six-row table may communicate the answer better than any chart.
Label the meaning, not just the number
Include units, time range, and the population being measured. “Records per day in a sample extract” is more useful than “Volume.” If the figures are synthetic, say so next to the figure rather than hiding the qualification in a footer.
The small chart on the home page uses six synthetic daily totals. Its bars have a zero baseline, and the underlying values are available as a CSV. They do not indicate the health or performance of a real service.
Keep comparisons honest
- Use a zero baseline for bars when their lengths encode magnitude.
- Preserve time order and show gaps rather than silently connecting missing periods.
- Avoid relying on color alone; include labels or other distinctions.
- Separate an observed association from a causal explanation.
If an axis is narrowed to reveal a small change, make that choice conspicuous. If values are estimates, consider showing a range rather than implying more precision than the source provides.