Showing change over time
A line graph shows how something changes over time: temperature through the day, a plant’s height each week, or money saved each month. Each measurement is a data point, and the points are joined with straight line segments.
Parts of a line graph
| Part | Job |
|---|---|
| Title | Tells what the graph shows |
| Horizontal axis | Usually time (hours, days, months), in order |
| Vertical axis | The quantity measured, with an even scale |
| Data points | The actual measurements |
| Lines | Join the points to show the change |
Reading the story in the line
| Line goes… | Meaning |
|---|---|
| Up | Increasing |
| Down | Decreasing |
| Flat | No change |
| Steeply | Changing quickly |
| Gently | Changing slowly |
The overall direction is called the trend.
Example: temperature in a day
| Time | 6 a.m. | 9 a.m. | Noon | 3 p.m. | 6 p.m. |
|---|---|---|---|---|---|
| Temperature (°C) | 8 | 14 | 20 | 22 | 16 |
Plotting these and joining them shows:
- A steep rise from 6 a.m. to noon.
- A gentle rise from noon to 3 p.m. (the warmest time).
- A fall from 3 p.m. to 6 p.m.
Reading between the points
Because the line joins the points, you can interpolate: estimate values in between. At 10:30 a.m. (halfway between 9 a.m. and noon), the temperature was about 17 °C.
Be careful: this is only an estimate. Interpolating makes sense for things that change smoothly, like temperature.
When to use a line graph
| Use a line graph | Use a bar graph |
|---|---|
| Data changes over time | Comparing separate categories |
| Height of a plant each week | Favourite fruits |
| Population each year | Number of pets per student |
Making a line graph
- Put time on the horizontal axis, evenly spaced.
- Choose an even scale for the vertical axis.
- Plot each data point.
- Join the points in order with a ruler.
- Add a title and label both axes with units.
Worked examples
From the temperature table:
Example 1: When was it warmest?
3 p.m. (22 °C).
Example 2: How much did it rise from 6 a.m. to noon?
20 − 8 = 12 °C.
Example 3: Between which two times did it fall?
3 p.m. and 6 p.m.
Common mistakes
- Uneven time spacing on the horizontal axis.
- Joining points out of order.
- Using a line graph for categories that have nothing in between, like types of pets.
- Reading the wrong axis.
Try it yourself
A plant’s height: Week 1: 2 cm, Week 2: 5 cm, Week 3: 9 cm, Week 4: 10 cm.
- During which week did it grow the most?
- What is the trend?
- Estimate the height halfway through Week 2 to Week 3.
- Why is a line graph a good choice here?
Answers: 1) Week 2 to Week 3 (+4 cm) 2) increasing 3) about 7 cm 4) It shows change over time.
Quick recap
- A line graph shows change over time.
- The slope of the line shows the trend and how fast things change.
- You can interpolate between data points for smooth data.