Chart guide
A plain guide to reading and drawing charts.
Charts are arguments in pictures. Most of their meaning comes from a few decisions: the question, the chart type, the scale, the labels and the words around them. This guide covers each one in order.
Step one
Start with the question, not the chart type.
Most weak charts begin with a tool. Someone opens the chart menu, picks the one that looks finished, and fills it with whatever is handy. Reverse the order. Write the question first, in one sentence, and check that it is something a picture can answer.
A good question names what is being compared and over what period. "How did the spring workshops compare with the autumn ones?" can be drawn. "How are the workshops doing?" cannot, because nobody agrees on what doing well means.
Step two
Bars and lines answer different questions.
Bars compare separate things
Use bars when the categories have no natural order between them, such as branches, committees, programs or product lines. Each bar stands alone, so the reader compares heights and nothing else.
Lines show movement
Use a line when the horizontal axis is time, such as weeks or months, and you want the reader to follow change from one point to the next. A line between unrelated categories suggests a path that does not exist.
Pie charts are not covered here. They ask readers to compare angles, which most people do poorly.
Step three
Start a bar chart's axis at zero.
A bar encodes its length. If the bottom of the axis sits above zero, small differences become large shapes, and the reader sees a story the numbers do not support. The two drawings below use the same three values. Only the starting point of the axis has changed.
Illustration only. The shapes are invented to show the effect of a baseline. They are not figures from any group.
Step four
Label what the reader needs.
A chart should explain itself in the time it takes to glance at it. Four labels do most of the work.
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A title that states the finding
Say what the chart shows, not what the data is called. A title such as "Renewals rose after the spring drive" says more than "Membership figures". Keep it short enough to read in one line.
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Axis names with units
Name each axis and give its unit. "Visits per week" is clear. "Visits" is not, because the reader cannot tell whether the figure is a total or a rate.
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Direct labels
Write values and category names on or beside the marks where you can. A separate legend makes the reader look back and forth, and some will stop looking.
A fourth label does its work at the foot of the picture: a source line that says where the numbers came from and the period they cover. It is the cheapest way to earn trust.
Step five
A blank is not a zero.
A blank cell means nobody recorded a value. A zero means someone counted none. They look alike in a spreadsheet and mean different things in a chart. In a line chart, leave a gap where a value is missing, and say how many values are missing in the caption.
When a missing value is drawn as zero, the line drops toward the floor and the reader concludes that activity fell. Often it simply was not recorded.
Step six
Use color to separate, never to decorate.
Color is good at telling groups apart and poor at carrying meaning alone. Some readers cannot tell certain colors apart, and printed charts often lose color altogether. Pair color with labels or with a different mark.
Keep the contrast between a bar and its background at least 3 to 1. Choose one main color for the data and a neutral for everything else. If you need three or more groups in a single chart, ask whether three small charts would read more clearly.
Step seven
Test the picture on a stranger.
Show the finished chart to someone who did not make it. Give them ten seconds, then ask what changed and what it means. If their answer differs from yours, the title, scale or labels are doing the wrong work. That is worth fixing before anyone else sees it.
Words we use
A short glossary.
- Axis
- The line along which values are measured. Most charts have one or two.
- Baseline
- The value a bar grows from. For bars it should normally be zero.
- Scale
- The range of values an axis covers, and the steps between its tick marks.
- Series
- One set of related values, drawn as a run of bars or a single line, usually given its own name.
- Caption
- The words beside a chart that say what it shows, what changed and what did not.
- Source line
- The note that names where the numbers came from and the period they cover.
This guide describes how to read and draw charts. It is not financial, legal, or medical advice. A chart built from money, health or legal records should be checked by a qualified person before anyone acts on it.
Practice
Try the rules on your own numbers.
The chart builder applies the zero baseline by default, labels every value and lists the numbers beneath the picture. Use it to test a draft before it goes to your board.