I have found a discrepancy between the matlab specification and
the octave implementation of the zplane function, found in the signal
The octave implementation does not give any output values, but matlab does:
/[hz,hp,ht] = zplane(___) returns vectors of handles to the zero lines, hz,
and the pole lines, hp.
ht 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./