Level 6 · Reading a chart honestly

A chart persuades faster than a table, which is why a careless one does more damage. Three traps cover most misleading charts in business decks, and you can spot each in seconds.

Trap 1: the truncated axis

The length of a column or bar is read as its value. If the value axis starts above zero, every column loses the same chunk at the bottom, and the differences that remain look much bigger. Here, September and October revenue were 143,378.66 and 174,599.43. On an axis from $140,000, the visible parts are 3,378.66 and 34,599.43: about ten times apart, for a real rise of 21.8%.

Excel sets the axis automatically, and when the values are close together it often starts above zero. Fix it with a right-click on the value axis → Format Axis → Bounds → Minimum = 0. Line charts are different: a line shows change, not size, so a line axis may start above zero, as long as the axis labels make the scale clear.

Trap 2: a pie with too many slices

A pie works for a few parts of one whole, like the six categories in level 1. With twenty slices, the small ones turn into slivers that can’t be told apart, and the reader is left matching colours to a long legend. Two more pie traps: slices that don’t add up to a real whole (percentages from different questions), and two pies side by side, which can’t show which total is bigger. Use a bar chart sorted from largest to smallest instead, or keep the top five and group the rest as “Other”.

Trap 3: correlation sold as cause

In level 5 cheaper Kitchen products sold more units, with a correlation of -0.69. A headline like “lower prices drive sales” goes further than the chart can. Both price and units depend on what the product is: a bowl set is cheap and bought often, a carafe is dear and bought once. A scatter plot shows that two numbers move together. Only a test (change one thing, keep the rest equal) shows that one causes the other.

A checklist before a chart goes out

• Does the title say what, where and when, in plain words?
• Do column and bar axes start at zero?
• Are both axes labelled, with units?
• Does a pie have a handful of slices that add up to one whole?
• Is every claim in the headline something the numbers support on their own?

Common mistakes

• Comparing heights instead of numbers. Read the values or the data labels, then compute the change.
• Fixing the axis on the slide but not in the file, so the next refresh brings the old axis back. Set the minimum in Format Axis.
• Hiding the axis to make the chart “cleaner”: then nobody can see that it’s truncated.