The reader is comparing lengths, so the lengths must be in proportion to the numbers.
Drawing a bar diagram
| Step | What to do |
|---|---|
| 1 | Draw two axes: categories along the bottom, frequency up the side |
| 2 | Choose a scale that fits the largest frequency, marked evenly |
| 3 | Make every bar the same width, with equal gaps |
| 4 | Label both axes and give the diagram a title |
If the axis starts at 950, a bar of 1100 looks three times a bar of 1000. The numbers are correct and the picture is still a lie.
A wider bar looks more important even when its height is unchanged, because the reader sees the whole area.
Kinds of bar diagram
A double bar diagram puts two related sets side by side and needs a key. Bars can run horizontally when the category names are long.
Bars with gaps mean separate categories. Bars that touch mean a continuous scale with no gap in it.
Check the scale, then check where the vertical axis begins. Those two things take ten seconds and prevent almost every wrong conclusion.
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