solution

Was there much collinearity in the original four-variable model? Was the collinearity reduced much by dropping two variables?

Partial residual plots, which are also called component plus residual or cr plots, can be constructed using the function cr.plot in the car package. Run

Besides dashed least-squares lines, the partial residual plots have solid lowess smooths through them unless this feature is turned off by specifying smooth=F, as was done in Figure 12.8. Lowess is an earlier version of loess. The smoothness of the lowess curves is determined by the parameter span, with larger values of span giving smoother plots. The default is span = 0.5. In the code above, span is 0.8 but can be changed for all four plots by changing the variable sp. Lowess, loess, and span are described in Section 21.2.1. A substantial deviation of the lowess curve from the least-squares line is an indication that the effect of the predictor is nonlinear. The default color of the cr.plot figure is red, but this can be changed as in the code above.