Time Series Graphs
What is a time series graph?
- A time series graph is sometimes called a line graph (which is different to a line chart)
- A time series graph shows how a quantity (continuous data) changes over time
- e.g. How the outside temperature changes during a week
- 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
- e.g. 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
- Be careful if both axes are both numerical with with similar values
- The data collected should be able to be plotted along the horizontal axis at regular intervals
- otherwise the graph could be misleading
- 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/line
- e.g. one line for car emissions, 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 general patterns in the data as well as specific points
- If a question asks you to interpret/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
- unusual 'one-off' readings - 'spikes' or 'dips'
- e.g. On 4 July 1990, around 26 million people watched the men's England football team lose on penalties to West Germany; shortly afterwards there was a sudden increase ('spike') in electricity use as many viewers went to put the kettle on!
- 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' line, like /)
- or a decrease ('downhill' line, like \)
- If a time series graph has points plotted at irregular intervals then
- the gradients of the lines will not be accurate
- it will be more difficult to determine some of the features of the data set
- 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
Examiner Tip
- If you are asked to describe or interpret a given line graph then use it carefully
- Draw lines on the given graph from the correct time or measurement to ensure an accurate reading
- Highlight any particular points that you mention in your description
Worked example
In a two hour charity 'dance-a-thon' dancers can join and leave the dance floor as they choose.
The number of dancers on the dance floor is recorded every 15 minutes.
The times-series graph below shows the data for the first two hours of the 'dance-a-thon'.
At 1 h 45 m and 2 h the number of dancers were 3 and 6 respectively.
Add these two recordings to the time-series graph.
Plot the points 105 minutes (1 h 45 m) on the time axis against 3 on the number of dancers axis and 120 minutes against 8 dancers.
Use the time-series graph to find
the time at which there were the fewest number of dancers,
the two times at which the number of dancers remained the same.
Look for the lowest point on the graph, including those added in part a).
The lowest point is (105, 3)
The final answer is the time.
105 minutes (1 h 45 m)
The keyword here is remain - a horizontal line would indicate the number of dancers remaining the same.
There is a horizontal line between 45 and 60 minutes
The number of dancers remained the same at 45 m and 60 m (1 h)
Comment on the general pattern of the number of dancers shown by the time-series graph.
Apart from the last point, and where it remained the same, the number of dancers decreased over the two hours.
In general, the number of dancers decreased during the first two hours of the 'dance-a-thon'
Avoid speculating on why this might be - e.g. DO NOT write "people get tired".
If a question wants you to do this, it will use phrases like "suggest a reason for ..." or similar.
