In reply to Rick Wicklin.

Many thanks for your above explanations.
I notice that, in the Fisher's iris data you are using as an example, you select only continuous variables (for both the dependent and the independent variables).
What is the validity and interpretation of partial leverage plots when the full model includes dummies as explanatory variables ("individual" dummies as well as dummies belonging to a set of dummies standing for a more than two categories nominal variable).
And how to cope with partial leverage plots when the model specification includes quadratic terms (X and X²) and/or interaction terms between, say, two variables? Are these plots still usable ?