Choose the graph after identifying what the numbers represent and what comparison matters.
Bar charts compare separate categories. Keep a zero baseline when bar length represents magnitude. A pie chart represents mutually exclusive parts of one whole; the percentages must total 100%. Twelve, six and two journeys out of 20 give 60%, 30% and 10%, or angles of 216, 108 and 36 degrees.
Line graphs show an ordered sequence, often time. Label the interval and units; joining points implies a progression, not proof of what happened between observations. Use evenly spaced positions only for evenly spaced times, or a numerical time axis when gaps differ.
Scatter graphs pair two variables and help show association. Each point must use x and y values from the same observation. Do not join scattered cases as if they were a time series. Describe the overall relationship and exceptions rather than assuming every point lies on a line.
Tables preserve exact values and can show more variables than one chart. Give clear headings and units, and retain sample sizes. Graphs make patterns easier to see; tables make precise checking easier. Avoid truncated scales, inconsistent units or decorative shapes that exaggerate differences.
Step by step
Identify the comparison
Categories, whole, sequence or paired relationship?
Draw and label
Use a suitable scale, title, units and key; retain the source and date.
Check meaning
Confirm that the display has not hidden denominators or implied a false connection.
Worked example: A pie chart that would mislead
Residents may report both using buses and walking. A pie of these overlapping responses is invalid because they are not exclusive parts of one whole. A bar chart can show the number reporting each mode, with a note that multiple answers were allowed.
Watch out for this
Any percentages can be put into a pie chart.
The categories must be parts of the same whole and must not overlap. Separate survey questions need a different display.
Check your understanding
Which graph best examines the relationship between route distance and journey time for 20 journeys?
- A pie of all distances and times.
- A line joining residents in name order.
- A scatter graph with one point per journey.