Visual Evidence and Data Stories
Visual Evidence and Data Stories
Begin with the reader’s task
| Task | Usually effective | Common failure |
|---|---|---|
| compare categories | sorted dot or bar plot | 3D perspective and area distortion |
| trace time | line plot with event annotations | truncated period or dual axes |
| inspect distribution | histogram, ECDF or box/violin plus points | mean-only bar |
| inspect relationship | scatterplot with groups and uncertainty | overplotting or fitted line without raw data |
| compare actual with target | aligned point/interval display | traffic-light colour without scale |
Position on a common scale is generally decoded more accurately than angle or area. Cleveland and McGill’s graphical-perception experiments provide the foundation; the lesson is to match encoding to comparison, not to ban one chart mechanically.
One display, one decision question
- Weak title: Delivery performance
- Decision title: Evening slots account for 71% of delays above 30 minutes; test capacity before changing all promises
The second title states the pattern and the proposed next step. It must still show:
- period and population;
- denominator and sample size;
- comparison or baseline;
- uncertainty or data limitation;
- annotation for operational changes.
Do not hide the scale
A bar from 48% to 52% can make a four-point change fill the page. For magnitude comparisons, bars normally begin at zero. A line chart may use a narrower axis to reveal change, but label the range and provide context.
Dual axes can imply a relation by choosing convenient scales. Prefer aligned panels or index both series to a documented baseline when the comparison is about relative change.
Display uncertainty where action depends on it
Suppose Store A late rate is 8% from 100 orders and Store B is 7% from 10,000. The one-point ranking hides radically different precision.
Show:
- interval estimates or posterior distributions;
- sample size and exposure;
- forecast bands across horizon;
- scenario ranges for assumptions;
- missing-data and revision indicators.
Uncertainty is not a decorative haze. It should help the reader decide whether alternatives are meaningfully different. Research on uncertainty-visualisation evaluation also warns that visual accuracy is not the same as decision quality.
Accessibility and integrity checklist
- title states the question or conclusion;
- text and table provide the key values;
- colour is redundant with labels, shape or line style;
- palette remains distinguishable with colour-vision differences;
- labels use units and plain language;
- source, cutoff and exclusions are visible;
- missing periods are gaps, not connected lines;
- annotations distinguish observed events from interpretation.
A three-sentence data story
- Context: “Across 24 completed weeks, 7.8% of accepted deliveries were late.”
- Evidence: “The rate was 4.1% before 17:00 and 13.6% after 17:00; evening orders also had larger baskets and longer routes.”
- Actionable uncertainty: “Pilot one extra picker in two comparable evening zones; the aggregate split alone does not identify a staffing effect.”
The story does not pretend the chart has already answered the causal question.
Quick check
A chart highlights the top five stores by complaint count. What essential exposure may be missing?