Line Graphs - Showing Change Over Time
A line graph plots data points on a grid and connects them with straight lines. Line graphs are used to show how a value changes over time. The direction and steepness of the line tells you whether the value is increasing, decreasing, or staying the same.
Key Features of a Line Graph
- Horizontal axis (x-axis) – usually shows time (days, months, years).
- Vertical axis (y-axis) – shows the measured quantity.
- Plotted points – each data value is marked as a dot.
- Line segments – dots are joined in order to show the trend.
- Title and axis labels – always required.
Reading Trends
| Line direction | What it means |
|---|---|
| Rising (upward slope) | Value is increasing |
| Falling (downward slope) | Value is decreasing |
| Flat (horizontal) | Value is not changing |
| Steep slope | Rapid change |
| Gentle slope | Slow change |
Worked Examples
The table shows average temperature (C) over six months. Jan: 3, Feb: 5, Mar: 9, Apr: 13, May: 17, Jun: 21. Answer the questions.
What is the overall trend? Temperature rises steadily – the line slopes upward from left to right.
What is the increase from January to June? 21 − 3 = 18 C.
In which month is the increase greatest? From May (17) to June (21): rise of 4 C – same as Mar to Apr and Apr to May. All equal at 4 C.
A line graph shows a plant's height (cm) over 5 weeks. Wk 1: 2, Wk 2: 5, Wk 3: 9, Wk 4: 9, Wk 5: 12. Describe the growth.
The plant grows rapidly at first, stays the same height in week 4 (flat line), then grows again in week 5. Overall growth = 12 − 2 = 10 cm.
Key Takeaways
- Line graphs show change over time – ideal for continuous data.
- The steeper the slope, the faster the change.
- A flat line means no change; a rising line means increase; a falling line means decrease.
- Both axes must be labelled, start at a suitable point, and use equal intervals.
Practice Questions
- Plot a line graph for monthly rainfall (mm): Jan 40, Feb 35, Mar 50, Apr 60, May 45, Jun 30.
- Using your graph, describe the overall trend between January and April.
- A shop's weekly sales are: Mon 80, Tue 95, Wed 70, Thu 110, Fri 130. Which day showed the greatest increase from the previous day?
- Why is a line graph more appropriate than a bar chart for showing temperature over time?
- A line graph shows two students' test scores over 5 tests. Student A: 50, 55, 60, 58, 65. Student B: 70, 65, 63, 68, 70. Who is improving more consistently?