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 health care center data, the plot shows the month when the center discharged each patient. The plot gives an approximation of the residuals over time.  Discharges for other reasons are more common in the 4th and 6th months. An exploration of whether the month that the center discharges a patient affects how the center discharges the patient is potentially valuable.