Scatter graphs
Scatter graphs are used to represent and compare two sets of data.
By looking at a scatter graph, we can see whether there is any connection (correlation) between the two sets of data.
Example
Matt sells ice-creams at outdoor events. 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 (\(^\circ C\)) | 21 | 26 | 15 | 24 | 18 | 29 | 20 | 27 | 23 | 17 | 30 | 19 |
| Number of ice-creams sold | 70 | 86 | 50 | 80 | 58 | 96 | 66 | 92 | 74 | 54 | 100 | 62 |
| Temperature (\(^\circ C\)) |
|---|
| 21 |
| 26 |
| 15 |
| 24 |
| 18 |
| 29 |
| 20 |
| 27 |
| 23 |
| 17 |
| 30 |
| 19 |
| Number of ice-creams sold |
|---|
| 70 |
| 86 |
| 50 |
| 80 |
| 58 |
| 96 |
| 66 |
| 92 |
| 74 |
| 54 |
| 100 |
| 62 |
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 graph.
Plotting a scatte rgraph 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.
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:
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.
