Reading a chart honestly

A chart is an argument. It has chosen what to show, what to leave out and how to draw it. Most charts are honest. Some mislead by accident and a few on purpose. Either way, the skill of reading one carefully is worth more than the skill of making one.

Five questions for any chart

Before you believe a chart, ask these in order.

1. What is being measured? Sales, orders, customers? In what unit and currency?
2. Over what period? One week or five years? Does it include the latest incomplete month?
3. Compared with what? Last year, the budget, another café? A number alone says very little.
4. Where does the axis start? At zero, or somewhere that makes a small change look large?
5. How much data is behind it? “Satisfaction up 50%” could be two customers becoming three.

If the chart cannot answer these, the title, labels or footnote are missing something.

Common ways charts mislead

The cut-off axis. A bar chart that starts at 95 instead of 0 turns a rise from 96 to 98 into a bar twice as tall. For bars, the axis should start at zero. For lines it may start higher, but look at the numbers on the axis before reacting to the slope.

Two axes on one chart. With sales on the left axis and customers on the right, the two lines can be made to cross, meet or diverge just by changing the scales. The crossing point means nothing.

The running total. A cumulative line always goes up, even in a terrible month. If a chart can only rise, it tells you little about how things are going.

The chosen window. Start the chart just after a bad month and everything looks like growth. Ask what the months before looked like.

Percentages without the base. “Returns down 40%” sounds good. From ten to six, in a business that sells ten thousand items, it barely matters.

The right chart for the question

You do not need many chart types. Four cover most needs.

Question	Use
How do these compare?	Bars, sorted from largest to smallest
How has it changed over time?	A line
What is the exact figure?	A table, or one large number
How do two measures relate?	A scatter of points

Pie charts work for two or three slices and fail beyond that, because the eye is poor at comparing angles. If you need to compare more than three parts, use bars.

Good habits when you make one

• Give the chart a title that states the finding: “Old Town overtook Harbour in March”, not “Sales by café”.
• Label the axis with the unit.
• Show the date the data runs to.
• Sort bars so the pattern is obvious.
• Remove anything that carries no information: borders, shadows, a legend for a single series.

Try it

Find one chart in a report, a news story or a presentation this week. Run the five questions on it. Write down which one the chart answers badly. Then imagine the one change that would fix it.

Next: where the data behind the chart came from, and why two reports can disagree.