wn=[0.1:0.1:1]; zet=0.707; 
w=logspace(-1,1,100); mm=[]; pp=[];
for i=1:length(wn)
   nn=wn(i)^2; dd=[1,2*zet*wn(i), wn(i)^2];
   [m,p]=bode(nn,dd,w); pp=[pp,p]; mm=[mm,m];; 
end
subplot(211); semilogx(w,20*log10(mm)), 
subplot(212); semilogx(w,pp)