Picking the chart type from the claim — Data Viz Module 3

There are an infinite number of charts you can draw and a small finite number of claims they typically argue. The skill is in matching the claim to the chart, not the other way around. Most analysts start with the data and pick a chart that 'shows' it. Strong analysts start with the claim and ask which chart proves it.

The six claim types

Comparison across categories: how does A compare to B, C, D? → Horizontal bar chart, sorted descending.
Ranking: who is first, second, third? → Sorted horizontal bar; for many items, a slope chart or dumbbell chart.
Composition of a whole: what fraction of total is A? B? C? → Stacked bar (preferred); pie only when 2-3 slices and the message is dominance.
Distribution: how is X spread? → Histogram, density plot, or boxplot. For comparing distributions across groups, side-by-side boxplots or ridge plots.
Correlation: how does X move with Y? → Scatter plot, with a trend line if the claim is about strength.
Change over time: how has X moved? → Line chart for trend, area chart for cumulative, slope chart for two-point comparison.

The horizontal bar default

When in doubt, horizontal bars sorted descending beat any other chart for category comparisons. Long category labels are readable, the eye scans top-to-bottom naturally, and order makes magnitude obvious. Vertical bars are for time-on-x; horizontal for everything else.

When no chart is right

Three numbers? Write the three numbers in a sentence. A single comparison? Make it a big stat-card with one number. A precise table of values someone might look up? Use a table. The chart-by-default reflex is a tic; the goal is communication, not visualisation.

Type-mismatch traps

Pie chart with 9 slices (no one can compare angles past 4 segments). Line chart for categorical x-axis (a line implies temporal continuity; using it for 'product A, B, C, D' is wrong). Bar chart with truncated y-axis (visually exaggerates small differences and is a credibility leak). Dual-axis line chart (almost always misleading — pick the more important axis and reference the other in text).

The default-chart matrix

text

CLAIM TYPE         DEFAULT CHART              WHEN TO DEVIATE
────────────────   ────────────────────────   ──────────────────────
Category compare   Horizontal bar (sorted)    Heat map for 50+ items
Ranking            Horizontal bar             Slope chart for 2 periods
Composition        Stacked bar                Pie for 2-3 dominant slices
Distribution       Histogram or boxplot       Violin for shape + density
Correlation        Scatter plot               Hex bins for >5000 points
Change over time   Line chart                 Area for cumulative

Memorise this table. 80% of analyst charts can be picked from it in five seconds.

Exercise

For each of the following claims, pick the right chart type and explain why: (1) 'Africa's banking penetration trails Asia by 30 points'. (2) 'Customer acquisition cost has tripled since 2022'. (3) 'Mobile money adoption is bimodal — heavy users and non-users, with little middle ground'. (4) 'The new packaging caused a 7% lift in conversion'.