Drawing scattergraphs

Scattergraphs are used to represent and compare two sets of data.

By looking at a scattergraph, we can see whether there is any connection (correlation) between the two sets of data.

Example

Matt sells ice-creams at outdoor events. He often buys too much or too little ice-cream from the wholesalers. This means he does not make as much profit as he would like.

He decides to record how many ice-creams he sells over a number of days, to see whether there is a link between the temperature and the number of ice-creams sold.

Here are his results:

Temperature ( ^\circ C)212615241829202723173019
Number of ice-creams sold7086508058966692745410062

When the temperature is low, the number of ice-creams sold is also low. When the temperature is high, the number of ice-creams sold is high.

It is easier to judge the results by looking at a scatter diagram.

Plotting a scattergraph is just like plotting coordinates.

The temperature is on the horizontal axis and the number of ice-creams sold is on the vertical axis.

The points to plot are (21, 70), (26, 86), (15, 50) etc.

Scatter diagram showing positive correlation between temperature and number of ice creams sold.

Note the jagged lines close to the origin. These indicate a broken scale. A broken scale is used when values close to 0 are not required.

For example:

A broken scale graph.

In this case, we only needed to start the horizontal axis at 15, and the vertical axis at 50.

Care must be taken to use the broken scale appropriately as it can sometimes be misleading.