After building a generalized additive model (GAM) using
mgcv package, we can use the
plot function to visualize the smoother, like:
plot(M1, resid = TRUE)
resid = TRUE argument ensures that residuals are added to the figure.
My question is: why this plot (to show the shape of smoother) is related to RESIDUAL? I mean, what we are interested should be the relationship between smoother and response variable, not residual. Then why residual occur in this smoother's plot?
In my immature understanding, "residual" is used in plots of model validation, e.g., residuals versus fitted values, residuals versus covariates.