Plotting Scatter Graphs

Grade 3

Setting up the axes

A scatter graph shows paired data — two measurements taken from the same subject (e.g. height and shoe size for the same person). One variable goes on the x-axis and the other on the y-axis.

If one variable is being used to predict the other, the predicting variable (called the independent variable) goes on the x-axis. If there's no obvious direction, either variable can go on either axis.

Label both axes with the variable name and its units. Choose a scale that uses most of the grid — a scale that squashes all your points into one corner makes patterns hard to see.

Plotting the points

EXAMPLE

Plot this paired data on a scatter graph: (2, 5), (3, 7), (5, 8), (6, 11), (8, 13).

Set x-axis from 0 to 9 (or 10), y-axis from 0 to 14 (or 15). For each pair, find the x-value on the horizontal axis, then move up to the y-value and mark a cross. Five crosses total. Five points correctly plotted at the given coordinates.

Common mistakes

The most common error is swapping the coordinates — plotting (5, 3) when the data says (3, 5). Always read across to x first, then up to y.

Use a cross (×) to mark each point, not a dot. Dots are easy to miss; a cross is unambiguous and easier for the examiner to check.

Try this one

A sports scientist records the daily temperature (in °C) and the number of cold drinks sold at a leisure centre. The scatter graph is shown below.

(a) Write down the word that best describes the correlation. [1]

Positive Negative No correlation

(b) On a day when the temperature is 22°C, estimate the number of cold drinks sold. [1]

(c) Give one reason why it would not be reliable to use this graph to predict the number of drinks sold on a day when the temperature is 50°C. [1]

Nova's hint:

This question covers scatter graphs, correlation and the reliability of predictions.

For part (b), trace up from 22°C to the trend of the points and read across.

For part (c), ask yourself: 'what would have to be true about the data for a prediction at 50°C to be trustworthy?'

Try to Solve it with Nova?