就是zplane默认有三个返回量：
用[HZ,HP,Hl] = ZPLANE(Z,P)给出
但是我不知道这三个量HZ,HP,Hl指代的是什么东西，请各位高手帮帮忙

zplane可以画零极点的分布图，但是[HZ,HP,Hl] = ZPLANE(Z,P)就不知道了。
>> help zplane
ZPLANE Z-plane zero-pole plot.
    ZPLANE(Z,P) plots the zeros Z and poles P (in column vectors) with the
    unit circle for reference.  Each zero is represented with a 'o' and
    each pole with a 'x' on the plot.  Multiple zeros and poles are
    indicated by the multiplicity number shown to the upper right of the
    zero or pole.  ZPLANE(Z,P) where Z and/or P is a matrix, plots the zeros
    or poles in different columns using the colors specified by the axes
    ColorOrder property.

    ZPLANE(B,A) where B and A are row vectors containing transfer function
    polynomial coefficients plots the poles and zeros of B(z)/A(z).  Note
    that if B and A are both scalars they will be interpreted as Z and P.

    [HZ,HP,Hl] = ZPLANE(Z,P) returns vectors of handles to the lines and
    text objects generated.  HZ is a vector of handles to the zeros lines,
    HP is a vector of handles to the poles lines, and Hl is a vector of
    handles to the axes / unit circle line and to text objects which are
    present when there are multiple zeros or poles.  In case there are no
    zeros or no poles, HZ or HP is set to the empty matrix [].

    ZPLANE(Z,P,AX) puts the plot into the axes specified by the handle AX.

等待高手出现……