Section: Handle-Based Graphics
    zplane(z,p)
 where z and p are the zeros and the poles of the system
 stored as column vectors, or
    zplane(b,a)
 where a and b are the polynomial coefficients of the
 numerator and denominator stored as line vectors (roots is
 used to find the zeros and poles). The symbol 'o' represents
 a zero and the symbol 'x' represents a pole. The plot includes
 the unit circle for reference.
Contributed by Paulo Xavier Candeias under GPL