Create a grid in the x-y coordinate system. Most of the function
is a reshaped extract from kde2d() from the MASS package, by
Venables & Ripley (+ modifications)
TT.xy.grid(x, y, n = 25)
Julien Moeys [aut, cre], Wei Shangguan [ctb], Rainer Petzold [ctb], Budiman Minasny [ctb], Bogdan Rosca [ctb], Nic Jelinski [ctb], Wiktor Zelazny [ctb], Rodolfo Marcondes Silva Souza [ctb], Jose Lucas Safanelli [ctb], Alexandre ten Caten [ctb]