Matlab 6.x Howtos 

How to draw a Bode Plot and Superimpose another plot to it

• Requirements: You need the Control Systems Toolbox (to be able to use the bode command)

% This examples shows how to draw a bode plot
% and then superimpose to it another plot

% First define the system transfer function
  G=tf(10,[1 2 1]);
% Find its Bode Plot (note left hand side argument)
  [mag,pha,w]=bode(G);
% Plot the Magnitude and Phase Plots
 % Magninutde Plot
  figure(1)
  semilogx(w,20*log10(squeeze(mag)))  % note the squeeze function
 % label the plot
  title('|G(j\omega|')
  xlabel('Frequency [rad/sec]')
  ylabel('Magnitude')
  grid
 % Phase Plot
  figure(2)
  semilogx(w,(squeeze(pha)))
 % Label the plot
  title('Phase of G(j\omega')
  xlabel('Frequency [rad/sec]')
  ylabel('Phase [deg]')
  grid

% To superimpose another figure just "hold on" to it
% To add to the first figure, first select it
  figure(1)
 % now hold it 
  hold on
 % now add whatever you want
  semilogx([.1 1],[1 1],'or')
  semilogx([.1 1],[1 1],'-g')
% Now add something to figure  2
  figure(2)
  hold on
 % we will add a continuous green horizontal line at -90 deg.
 semilogx([.1 10],[-90 -90],'-g')
% Once done release the hold
 figure(1),hold off
 figure(2),hold off