Pole-zero Contour with GNV Octave

For circuit (below), R1 + R/ (RsC +1 )
simplifies to 2((s+3) / (s+1))

%pole zero contour plot
clear
[x,y] = meshgrid(-4: 0.15: 4);
s = x + iy; G = abs((2s + 6) ./ (s + 1)) + eps;
contour(x,y,G, 0:0.15:12)