Interpret line graphs
Theory
A line graph joins points to show how a measurement changes over time. Read up from a time to its value, follow the slope to see it rise or fall, and subtract two readings to find a change.
A line graph joins plotted points with straight lines to show how one measurement changes over time.
To read a value, find a time on the bottom axis, go straight up to the point, then across to the scale.
A line sloping up means the value is rising; sloping down means it is falling; a flat line means no change.
The steepest part is where the value changes fastest.
The slope of the line tells the story between the points.
| Between | Change | Slope |
|---|---|---|
| 6 am – 9 am | \(10\to15\) | rising |
| 9 am – 12 pm | \(15\to25\) | rising fast |
| 3 pm – 6 pm | \(30\to20\) | falling |
How to interpret a line graph
- Find the time on the bottom axis.
- Read straight up to the point, then across to the scale for its value.
- Look at the slope: up is rising, down is falling.
- Subtract two readings to find how much it rose or fell.
Read up from 12 pm to the point.
| 12 pm | \(=\) | \(25^\circ\) |
Subtract the two readings.
| \(30-10\) | \(=\) | \(20^\circ\) |
Subtract the later reading.
| \(30-20\) | \(=\) | \(10^\circ\) |
The steepest part is the biggest jump: \(15^\circ\) to \(25^\circ\).
| 9 am | \(\to\) | 12 pm |
Common pitfalls
Frequently asked questions
How do I read a value from a line graph?
Find the time on the bottom axis, read straight up to the point, then across to the scale. The number it lines up with is the value.
What does the slope of the line mean?
An upward slope means the value is rising, a downward slope means it is falling, and a flat line means it stays the same.
How do I find how much a value changed?
Subtract the two readings. The size of the rise or fall is the difference between the values at the two times.
Which part changes fastest?
The steepest part of the line. The biggest jump up or down between two points is where the value changes fastest.
Why use a line graph instead of a column graph?
A line graph is best for change over time, because joining the points shows how the value rises and falls between the readings.