Bar Charts: How to Draw, Read, and Worked Examples

A bar chart presents data using rectangular bars whose heights are proportional to their values. It is the right choice for comparing categories. The topic appears in Year 4 and continues through secondary statistics.

The Parts of a Bar Chart

How to Draw One

Test attendance data: Monday 20 students, Tuesday 35, Wednesday 25, Thursday 40.

1. Choose the scale first. The largest value is 40, so 0–40 in steps of 5 works well. Too tight a scale makes the chart absurdly tall; too loose and the differences vanish.
2. Draw both axes and label them, including units.
3. Draw the bars with equal widths and equal gaps between them.
4. Write the value above each bar so it need not be guessed from the height.

The vertical axis must start at zero. That is not a formality — see the next section.

Misleading Charts

This part is rarely taught at school and is the most useful for life.

Take sales of 100 and 110 units. With the vertical axis starting at 0, the two bars look almost identical — which is honest, since the gap is only 10%. But start the axis at 95 and the second bar looks three times taller than the first.

The data is truthful and the picture lies. This trick appears daily in advertising, business decks and the news. The first rule for reading any bar chart: check what number the vertical axis starts at.

A child trained to check the scale gains something far more valuable than a test mark.

Reading a Bar Chart

The questions that come up in exams:

• Highest and lowest value — the tallest and shortest bars
• Difference between two categories — subtract the values
• Total of all data — add every bar
• Mean — total divided by the number of categories, see how to calculate the mean
• Percentage of one category — value divided by total, times 100%

Bar Chart or Something Else?

The Mistakes That Actually Cost Marks

1. A vertical axis that does not start at zero. The number one cause of misleading charts, deliberate or not.

2. Bars of unequal width. A wider bar looks larger even at the same value, and that deceives the eye.

3. An inconsistent scale. The gap from 0 to 10 must equal the gap from 10 to 20.

4. Forgetting the unit on the vertical axis. A bare 40 could mean anything.

5. Estimating bar heights by eye when the values are printed. Always use the numbers.