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Choosing the right chart

What it is

Choosing a chart means naming the comparison the reader has to make, then picking the form that makes that comparison easiest. The data rarely dictates the form; the question does. Most chart mistakes are wrong-form mistakes, not drawing mistakes.

Why it matters

Different forms answer different questions, and a reader has one second to extract the answer. A pie chart of eight categories makes comparison nearly impossible, while the same data as a sorted bar chart takes a glance. Choosing well is the difference between a chart that informs a decision and one that is decoration with numbers attached.

Core ideas

Name the question, then the form

Write the comparison in a sentence, then look it up.

Question                                  Form
Which category is largest?                sorted bar
How has this changed over time?           line
Is this one of many, or exceptional?      dot plot or bar with a reference line
How do parts make up a whole?             stacked bar (or a table)
Is there a relationship between two?      scatter
What is the distribution?                 histogram

Bars for comparison, lines for change

Bars answer “which is more” and are read by length; lines answer “what happened next” and are read by slope. Using a line for unrelated categories implies a sequence that does not exist; using bars for a time series hides the trend.

Sort bars by value, not alphabetically

Alphabetical order makes the reader scan the whole chart to find the largest. Sorting by value turns the chart into a ranking.

Alphabetical:  Bristol, Leeds, London, Manchester, York
By value:      London 420, Manchester 310, Bristol 240, Leeds 180, York 90

Keep pies and donuts for two or three slices

A reader compares angles poorly. Two or three shares of one whole is about the limit; anything more should be a bar chart or a table.

Good pie:  mobile 60% / desktop 40%
Bad pie:   eight categories, six of them under 5%

A table is a legitimate answer

When the reader needs exact values, or the categories are many and unrelated, a well-formatted table beats any chart. Sorting and alignment do most of the work.

Practice

Re-form three charts

Take three charts from your product, write the question each is meant to answer, and re-form any where the form does not match the question.

Done when: each chart has its question written beside it and either matches it or has been replaced.

Sort one chart by value

Find a bar chart in alphabetical or arbitrary order and sort it by value. Note what becomes visible.

Done when: the largest and smallest values are readable without scanning the axis labels.

Common mistakes

  • A pie with many slices — angles cannot be compared and the chart is unreadable; fix: use a sorted bar chart or a table.
  • Lines connecting unrelated categories — the slope implies a trend that does not exist; fix: bars for comparison, lines only for time.
  • Alphabetical bar order — the ranking is hidden and the reader scans everything; fix: sort by value.
  • A chart where a table would do — exact values and many categories are both worse in a chart; fix: use a sorted table.
  • Choosing the form before the question — the chart answers something nobody asked; fix: write the question first and look up the form.

Go deeper

Worked examples

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