polys.plot() in mgcv but with added arguments zlim and rev.col and a wider set of choices for
scheme.polys.map(lm, z, scheme = "gray", lab = "", zlim, rev.col = TRUE, ...)lm."heat", "terrain", "topo", "cm" and "gray", indicating how to fill
the polygons in accordance with the value of z.pretty(z) otherwise the range provided will be used.FALSE then coloring scheme is not reversed.polys.plot in mgcv.