Root locus figure is bad plotted

[Beginner1]

Hi!,

I would like to know how to solve an issue I have with the root locus plot. 
This is my code

NUM{1,1}=[-15,-300];
DEN{1,1}=[1,0];
G_idref_vdcref = tf(NUM{1,1},DEN{1,1});

rlocus(G_idref_vdcref); 
hold on 
n_idref_vdcref=roots(NUM{1,1})
d_idref_vdcref=roots(DEN{1,1})
plot(real(n_idref_vdcref),imag(n_idref_vdcref),"co","markersize",15)
plot(real(d_idref_vdcref),imag(d_idref_vdcref),"rx","markersize",15)


And the resulting root locus plot is:
<http://octave.1599824.n4.nabble.com/file/t372684/rlocusplot1.jpg> 

what is obvious, incorrect, since in my  transfer fuction there is a pole at
s=0 and a zero at s=-20, and in the picture we see that the rlocus line goes
to instability and other areas that it is not supposed to go. If I zoom the
rlocus plot we see:
<http://octave.1599824.n4.nabble.com/file/t372684/rlocusplot2.jpg> 

Where the zero and the pole are correctly located, however the connecting
line between them is uncorrect. Any suggestion to solve this error?
Thanks.  



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