[R] How to insert gridlines in lattice density plot

[Giles]

Thank you Burt Gunter and David Winsemius,

I confirmed David's comment.
One must use type=c("p","g")
in order to get both the grid and points.

So, to use densityplot well,
we must be quite aware of the options
for panel.xyplot!
Cordially,
Giles Crane