Data
Line graphs & frequency diagrams
A line graph shows how something changes over time. The shape of the line tells the story — rising, falling or staying level.
What a line graph shows
A LINE GRAPH plots measurements and joins them with a line. It is used when the thing you measured changes CONTINUOUSLY — like temperature over time. The x-axis (across) usually shows time. The y-axis (up) shows the measurement. Joining the points is what makes it a line graph, and it means you can estimate values in between.
Describing the shape
Read the line from left to right and describe what it does, using words like UP, DOWN and LEVEL. Rising line → the measurement is increasing. Falling line → it is decreasing. Flat line → it is staying the same. Steeper line → it is changing faster. Describe each part in turn: 'the temperature rose for 15 minutes, then stayed level, then fell'.
Frequency diagrams
A FREQUENCY DIAGRAM is used for measurements that are grouped into intervals, such as heights from 120–130 cm. The bars touch, because the data is continuous — there is no gap between one group and the next. That is the visible difference from an ordinary bar chart, where the bars have gaps because the categories are separate.
Reading between the points
Because a line graph joins its points, you can estimate a value between two readings. If the temperature was 3 °C at 10 minutes and 5 °C at 20 minutes, it was probably about 4 °C at 15 minutes. This is an estimate, not a measurement — but it is a reasonable one, and it is exactly why the points are joined up.
Worked examples
Describing a rising line
- Read the graph from left to right.
- The line goes upwards.
Answer: The temperature is increasing
Describing a flat section
- Between 15 and 20 minutes the line is level.
- Level means no change.
Answer: The temperature stayed the same
Estimating between points
- At 10 minutes it was 3 °C; at 20 minutes it was 5 °C.
- 15 minutes is halfway between.
Answer: About 4 °C
Bar chart or frequency diagram?
- The data is heights grouped into intervals — continuous data.
- Continuous data means the bars touch.
Answer: A frequency diagram
Choosing the right graph
- The measurement changes over time and you want to see the trend.
- A line graph joins the points to show the change.
Answer: Use a line graph
Where you see this in real life
- A hospital chart tracking a patient's temperature through the day.
- A graph of daily rainfall showing which weeks were wettest.
- Plotting the temperature of cooling water every five minutes in a science experiment.
Key words
- Line graph
- A graph where plotted points are joined by a line.
- Frequency diagram
- A chart for grouped continuous data, with bars that touch.
- x-axis
- The horizontal axis, going across.
- y-axis
- The vertical axis, going up.
- Continuous data
- Data you measure, which can take any value.
- Trend
- The general direction of a change.