This graph plots the residuals versus another variable. The residuals should fluctuate in a random pattern around the center line. If the variable is already included in the model, use the plot to determine if you should add a higher-order term of the variable. If the variable is not already included in the model, use the plot to determine if the variable is influencing the response in a systematic way.

Use this plot to look for the following:

For the sales data, the residuals do not appear to be randomly scattered about zero. Evidence exists that there is a pattern in the residuals. The plot indicates that software sales may have a systematic effect on the computer sales. An outlier is also evident.