A line graph is used to show how a variable changes over time. It is particularly useful for identifying trends, patterns and changes across a period.

 

 

Constructing a Line Graph

To construct a line graph:

• choose appropriate axes
• place time on the horizontal axis
• place the variable being measured on the vertical axis
• choose a suitable scale for each axis
• plot each data point accurately
• join consecutive points with straight line segments

 

The points must be plotted in time order.

 

Both axes must be clearly labelled and include units where appropriate.

 

A consistent scale makes trends easier to see

 

 

Using Line Graphs

Line graphs are commonly used for:
• temperature over time
• distance travelled over time
• population change over years
• sales figures by month

 

They allow you to see whether values are:
• increasing
• decreasing
• staying constant
• changing at different rates

 

Steeper sections of the graph show faster change.

 

 

Interpreting Line Graphs

When interpreting a line graph, you should:
• describe overall trends
• identify maximum and minimum values
• compare different time periods
• comment on increases and decreases

 

Always refer to the graph when making statements.

 

For example, you might describe when the value rises most quickly or when it remains steady.

 

 

Intermediate Values

An intermediate value is a value read between two plotted points.

 

Whether intermediate values have meaning depends on the context.

 

Intermediate values may have meaning when:
• the variable changes continuously
• values could realistically exist between measured times

 

For example, temperature or distance travelled can be estimated between recorded times.

 

Intermediate values may not have meaning when:
• the data is discrete
• values only exist at specific times

 

For example, the number of people in a room counted once per hour may not be meaningful between counts.

 

Context determines whether intermediate values are valid

 

 

Common Errors to Avoid

Common mistakes include:
• using uneven or inappropriate scales
• joining points for data where intermediate values are not meaningful
• misreading values between points
• forgetting to label axes or units

 

Understanding the context prevents incorrect interpretation.

 

 

Key Points to Remember

Line graphs show how a variable changes over time.
Time is placed on the horizontal axis.
Points are plotted and joined in order.
Line graphs help identify trends and patterns.
Intermediate values may or may not have meaning depending on context.

 

Knowing when and how to use line graphs, and when intermediate values are valid, ensures data is represented and interpreted accurately.