Data Handling
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.
The line graph traces back to the same 1786 book that introduced the bar chart – William Playfair's Commercial and Political Atlas – where he used a rising and falling line to track Britain's trade balance with different countries over time, a task that would have been almost unreadable as a table of raw numbers. Today, the line graph's most famous descendant is the stock market chart: every ticking line on a trading screen, tracking a share price minute by minute, is applying exactly the same principle Playfair pioneered over two centuries ago, just updated thousands of times faster.
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
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.
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.