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Python & Django

Presenting Numbers So People Act on Them

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One message per chart

A chart showing six series over three dimensions communicates nothing. Decide what the reader should conclude, then show only what supports that conclusion.

If you cannot write the chart's message as a single sentence, the chart is not ready. That sentence should usually be the title.

Always compare against something

  • The same period last year, for seasonal businesses
  • The target, if there is one
  • The previous period, where seasonality is not a factor
  • Other categories, for composition questions

A number alone is not information. A number against a comparison is.

Remove everything that is not the data

  1. No decoration, gradients or three-dimensional effects
  2. No legend where direct labelling would do
  3. Gridlines faint or absent
  4. Axes starting at zero for bar charts, always
  5. Colour used to distinguish, not to decorate

Choose the right chart

QuestionChart
How has this changed over time?Line
How do these categories compare?Bar
What is the composition?Stacked bar, rarely pie
Is there a relationship?Scatter
What is the distribution?Histogram
A single important numberJust the number, large

That last one is underused. Sometimes the right visualisation is a number in large type with a comparison beside it.

Write the conclusion

Every chart in a business report should carry a sentence saying what it shows and what it implies. Readers should not have to work it out.

That sentence is also the test of whether the chart was worth including at all.

Frequently asked questions

Which library should we use?

Any of the established Python plotting libraries. Clarity of design matters far more than the tool.

Should charts be interactive?

For exploration, useful. For a report someone reads once, a static chart with a clear message is usually better.

How many charts in a report?

As few as make the point. Five charts that are read beat twenty that are skipped.

What about pie charts?

Rarely the best choice. People compare angles poorly; a bar chart is nearly always clearer.

Keep reading

Reports full of charts nobody reads?

Usually too many, each saying too little. One message per chart is the fix.

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