Picking the right graph
The best data display depends on two things:
- The type of data
- The purpose: what you want the reader to see
Types of data
| Type | Description | Examples |
|---|---|---|
| Categorical data | labels or groups | favourite sport, eye colour |
| Numerical data – discrete | counts, whole numbers | number of siblings, goals scored |
| Numerical data – continuous | measurements | height, time, temperature |
Choosing by purpose
| Purpose | Best display |
|---|---|
| Compare categories | bar graph, pictograph |
| Show parts of a whole | circle graph |
| Show change over time | line graph |
| Show a distribution | histogram, stem-and-leaf plot, box plot |
| Compare distributions | side-by-side box plots, back-to-back stem-and-leaf |
| Show a relationship between two variables | scatter plot |
| Show frequency of small discrete data | dot plot, bar graph |
Examples
| Question | Data | Best display |
|---|---|---|
| Which pizza topping is most popular? | categorical | bar graph |
| How did a city’s population change from 2000 to 2025? | numerical over time | line graph |
| How is the household budget split? | parts of 100% | circle graph |
| How are students’ heights spread? | continuous | histogram |
| Is arm span related to height? | two numerical variables | scatter plot |
| How do two classes’ test scores compare? | numerical, two groups | box plots |
Mismatches to avoid
- Line graph for categories: lines between "apple" and "banana" mean nothing.
- Circle graph when parts don’t add to a whole: e.g., "check all that apply" surveys.
- Bar graph with gaps for continuous data: use a histogram.
- 3D effects: they distort sizes and make reading harder.
Good display checklist
- Clear title
- Labelled axes with units
- Even scales, starting at 0 for bar graphs
- A legend if colours are used
- Source of the data
Worked examples
Example 1: Monthly rainfall over a year? Line graph (or bar graph by month).
Example 2: Percent of energy from each source? Circle graph.
Example 3: Times for 100 runners in a race? Histogram or box plot.
Common mistakes
- Choosing the most colourful graph instead of the clearest.
- Ignoring the data type.
- Overloading one graph with too many categories.
Try it yourself
Choose a display:
- The number of each type of tree in a park
- A stock price over 6 months
- Hours of sleep vs grade average for 40 students
- Distribution of 200 people’s ages
- How a $1,000 budget is spent
Answers: 1) bar graph 2) line graph 3) scatter plot 4) histogram 5) circle graph
Quick recap
- Match the data display to the data type and purpose.
- Categorical data: bar or circle graphs; numerical data: histograms, box plots, line and scatter plots.
- Check titles, labels and scales.