Time Series Graphs (AQA GCSE Maths): Revision Note

Time series graphs

What is a time series graph?

• A time series graph is sometimes called a line graph

• A time series graph shows how a quantity (continuous data) changes over time

  • e.g.  How the outside temperature changes during a week (shown below)

• Measurements of the quantity are taken at particular times

  • Measurements should be taken at regular time intervals

  • These are then plotted as points on a time series graph and joined together with straight lines

  • The straight lines help us to identify patterns and features in the data

• Time series graphs can show changes over short or long periods of time

  • e.g. Changes to the temperature of two chemicals for the first few minutes after they've been mixed

  • Or changes to the temperature of the earth over several years

How do I draw a time series graph?

• The horizontal axis (x-axis) will be the time axis

• The vertical axis (y-axis) will be the quantity being measured/recorded

• Plot the data as a series of points

• Join one point to the next, in order, with straight lines

  • Use a ruler

• Sometimes a time series graph may have more than one data set 

  • e.g.  one line for car emissions and one line for motorbike emissions

  • Plot one data set and join the points up before moving on to the second data set

    • This will ensure you do not muddle the points up

    • You could use crosses (×) for one set of points, and dots (•) for the other 

    • You could use different colours or dotted/dashed lines when joining the points up

    • Always include a key in such cases to make it clear which line is which data set

How do I use and interpret a time series graph?

• This involves looking at patterns in the data as well as specific points

• If a question asks you to interpret or describe a time series graph look for:

  • A general trend

    • e.g.  The rate of inflation may fluctuate (go up and down) but is generally going down over a decade

  • Seasonal differences

    • e.g. Sales are higher in the summer months than winter months

• Other things to look for

  • a horizontal line between points - no change (constant)

  • the steepest line (gradient) would indicate the greatest change

    • this could be an increase ('uphill' left to right, like /)

    • or a decrease ('downhill' left to right, like \)

  • Unusual 'one-off' readings - 'spikes' or 'dips'

    • e.g.  A spike in mobile phone network activity at midnight on New Year's eve

• For line graphs with two (or more) data sets, be clear about which line you are describing

  • Use the key

  • Double check which data set is 'higher' or 'lower' (or they may be equal) at a particular time

• Intermediate values on the graph (between two data points) may or may not have meaning

  • E.g. If you are heating water and measuring the temperature every 10 seconds

    • The value in between 20 and 30 seconds could be a reasonable estimate for the temperature at 25 seconds

  • If a line chart shows the maximum daily temperature for each day of the week

    • The value between Monday and Tuesday likely has no meaning

    • It is unclear what time this would represent

    • If it was modelled to represent midnight, it is unlikely that the temperature would be between the maximum temperatures of Monday and Tuesday