Reading graphs like a detective
Graphs are everywhere: news, sports, science, business. To interpret a graph well, you need to read what it shows and think critically about how it shows it.
Step 1: Read the basics
| Check | Ask |
|---|---|
| Title | What is this graph about? |
| Axis labels | What is being measured? In what units? |
| Scale | What does each grid line represent? Does it start at 0? |
| Legend | What do colours or symbols mean? |
| Source | Who collected the data? When? |
Step 2: Read the data
- Read values: find specific numbers.
- Compare: which is biggest or smallest? By how much?
- Find changes: how much did it increase or decrease?
Step 3: Look for patterns
- Trend: Is it going up, down or staying level?
- Peaks and dips: Where are the highs and lows?
- Outliers: Any values that don’t fit?
- Rate of change: Is it changing quickly (steep) or slowly?
Step 4: Draw inferences
An inference is a conclusion supported by the data.
Example: A line graph shows ice cream sales rising from May to August and falling in September. Inference: sales are higher in warmer months.
Be careful:
- Correlation isn’t causation. Two things rising together doesn’t mean one causes the other.
- Don’t predict too far. A trend may not continue.
Spotting misleading graphs
A misleading graph can make small differences look huge:
| Trick | Effect |
|---|---|
| Vertical axis doesn’t start at 0 | Small differences look big |
| Uneven scale | Distorts changes |
| 3D or pictures of different sizes | Areas exaggerate differences |
| Missing labels or units | You can’t tell what’s measured |
| Cherry-picked time range | Hides the full trend |
Example: Two bars show 50 and 52 sales, but the axis starts at 48. The second bar looks twice as tall, though the difference is only 4%.
Matching graphs to stories
A distance–time graph of a walk to school:
- Steady upward line → walking at constant speed
- Flat section → stopped (waiting at a light)
- Steeper section → running
Worked examples
Example 1: A bar graph shows Class A read 120 books and Class B read 90. How many more did Class A read, as a percent of Class B?
30 more; 30/90 ≈ 33% more.
Example 2: A graph’s y-axis runs from 95 to 100. Two values are 96 and 99. Why might it mislead?
The bars look very different, but the actual difference is small (about 3%).
Example 3: A line graph of a town’s population shows 5,000 (2000), 6,000 (2010), 7,000 (2020). Trend?
Increasing by about 1,000 per decade.
Common mistakes
- Skipping the axis labels and scale.
- Assuming the vertical axis starts at 0.
- Treating a trend as proof of cause.
Try it yourself
- What should you check first when reading any graph?
- A graph shows sales rising in winter for both hot chocolate and gloves. Does one cause the other?
- Name two ways a graph can be misleading.
- A flat section on a distance–time graph means what?
Answers: 1) The title, axis labels and scale. 2) No; both are linked to cold weather. 3) e.g. axis not starting at 0, uneven scale 4) not moving
Quick recap
- Interpret graphs by reading the title, axes, scale and data.
- Describe the trend and make careful inferences.
- Watch out for misleading graphs.