it would likely be easier to use the approach bode in Obtain Magnitude and Phase Data. (Remember to use the squeeze function with those, if necessary.)
This seems to work —
H = tf([1 0.1 7.5],[1 0.12 9 0 0]);
bodemag(H)
Ax = gca;
% get(Ax)
Lines = findobj(Ax, 'Type','Line');
% L1 = Lines(1);
L2 = Lines(2);
X = L2.XData
Y = L2.YData
.