grid.list
A list with as many components as variables describing the surface.
All components should have a single value except the two that give the
grid points for evaluation. If the matrix or data frame has column names,
these must appear in the grid list. Se
chull.mask
Whether to restrict the fitted surface to be on a convex hull, NA's
are assigned to values outside the
convex hull. chull.mask should be a sequence of points defining a convex
hull. Default is to form the convex hull from the observations if this
argument