Multiple bode plots fail on same figure

[John W. Eaton]

On 27-Jun-2008, Anon5710 wrote:

| It an easy to reproduce bug.
| Here's an example.
| 
| ********************
| clear all;
| [a1,b1] = butter( 7, [25, 41], 's');
| [a2,b2] = butter( 7, [49, 81], 's');
| bandpass_1 = tf( a1, b1);
| bandpass_2 = tf( a2, b2);
| figure(1);
| clf;
| bode( bandpass_1 )
| hold on;
| bode( bandpass_2 ) 
| ********************
| 
| The resulting figure has the phase plot of both system functions. But it
| only shows the magnitude plot of the last one to given. (in this case
| bandpass_2)
| 
| Are there any solutions to this?
| 
| I am using GNU Octave, version 3.0.1
| Compiled from source on my gentoo system. (I have instaled the various
| octave-forge-* packages.)

I think this problem is happening because Octave is not handling the
nextplot property correctly for figures (and the hold function is not
even setting it, which I think it should).  Here is a simpler example
that demonstrates the problem (no need for butter, tf, or bode):

  x = -10:0.1:10;
  subplot (2, 1, 1);
  plot (x, sin(x));
  subplot (2, 1, 2);
  plot (x, sin(x));
  hold ("on");
  subplot (2, 1, 1);
  plot (x, cos(x));
  subplot (2, 1, 2);
  plot (x, cos(x));

I don't know the proper fix for this.  I could use some help from
someone who understands just how the nextplot properties for figure
and axes objects are supposed to work.

Thanks,

jwe