plotfunc represents a complex expression E
(such that re(E) is not identically 0 on the discretization mesh)
by the surface z=abs(E) where arg(E) defines the color
from the rainbow. This gives an easy way to
see the points having the same argument.
Note that if re(E)==0 on the discretization mesh,
it is the surface z=E/i that is represented with rainbow colors
The first argument of plotfunc is E, the remaining arguments are the same as for a real 3-d graph (cf 6.2.2).
^2 with the same color for points having the same argument
We may specify the range of variation of x and y and the number of