\section{ZPLANE Zero-pole plot}

\subsection{Usage}

 This function makes a zero-pole plot of a discrete-time
 system defined by its zeros and poles. The various syntaxes
 are
\begin{verbatim}
    zplane(z,p)
\end{verbatim}
 where \verb|z| and \verb|p| are the zeros and the poles of the system
 stored as column vectors, or
\begin{verbatim}
    zplane(b,a)
\end{verbatim}
 where \verb|a| and \verb|b| are the polynomial coefficients of the
 numerator and denominator stored as line vectors (\verb|roots| is
 used to find the zeros and poles). The symbol \verb|'o'| represents
 a zero and the symbol \verb|'x'| represents a pole. The plot includes
 the unit circle for reference.
Contributed by Paulo Xavier Candeias under GPL
