removing units from plot - matlab

Question

I try to remove the Matlab-given units from this plot but I don't find a way:
figure(1)
hold on
set(gcf,'PaperUnits','centimeters',...
'PaperSize',[15 9],...
'PaperPosition',[0 0 15 9]);
pzmap(LB); sgrid; grid on; axis equal;
title('');
xlabel('\sigma [rad/s]')
ylabel('\omega [rad/s]')
hold off
After that commands the xlabel looks like this: \sigma [rad/s] (seconds^-1). The seconds comes with pzmap. How can I remove them?
I found, some strange behavour:
If generate code by the figure plot manager I get this:
% Create xlabel
xlabel('\sigma [rad/s] (seconds^{-1})','Units','pixels');
Why???
Now I get it - without pzmap/pzplot
pol = pole(sys)
figure(1)
plot(real(pol(:)),imag(pol(:)),'x')
title('');
xlabel('\sigma [rad/s]');
ylabel('\omega [rad/s]');
sgrid

Answer 1

pzmap is a high-level convenience function, but it's not the best choice for this (it's also stored in a folder of obsolete functions in R2013a, so it may get marked for official removal in the future). Instead, let's create an example plot using pzplot directly instead of pzmap. This is still a plot function that does a lot under the hood, but it returns a handle, h, to the plot:
sys = rss(3,2,2);
h = pzplot(sys);
sgrid;
axis equal;
We can via the options of a pzplot with getoptions:
p = getoptions(h)
To set the labels and units as you desire, you might try this, using setoptions:
p.Title.String = '';
p.XLabel.String = '\sigma';
p.YLabel.String = '\omega';
setoptions(h,p);
I believe that the units of 'seconds-1' that the plot displays is equivalent to the 'rad/s' that you want to specify. I know that the two look is very different (I prefer being specific about radians myself), but that's a disadvantage of using such a plot function that tries to do everything for you. If you wanted to remove the default string or add another option, you'd likely have to do some low level hacking. An easier way around, might be to use the "Generate Code..." command ("Generate M-File..." in older versions") under the "File" menu in the figure's toolbar and edit the plot labels there (there's also a programmatic option for this on the File Exchange). Or you could output to postscript and edit that.
Alternatively, you can use pzoptions to create a list of options to pass to pzplot or pzmap (undocumented in the latter case):
p = pzoptions;
p.Title.String = '';
p.XLabel.String = '\sigma';
p.YLabel.String = '\omega';
sys = rss(3,2,2);
pzplot(sys,p);
sgrid;
axis equal;
You'll see that that for some reason the text size is much smaller in this case. pzplot and pzmap must set the font size to 10 themselves. You could easily do this.
Fore more on customizing this and related Control toolbox plots, see this article.

Answer 2

After intense low-level digging, there is actually a pretty simple way to override the default behavior.
p = pzplot(sys);
p.AxesGrid.XUnits = 'rad/s';
p.AxesGrid.YUnits = 'rad/s';
Changes appear to take effect immediately. I have even tried setting the value to nothing, i.e.
p.AxesGrid.XUnits = '';
and it effectively removes the annoying parenthesis with the units. Technically, matlab creates a custom-class element they store under the name AxesGrid in the resppack.mpzplot class instance, with some standard LTI-behavior. You can probably work around some stuff by "injecting" a script with the same name as one of the standard library functions, so that it will be called instead, and change things in there, but this is the closest I have come to removing those annoying units in a few lines.
As a side info, the AxesGrid object is initialized in
...\controllib\graphics\#resppack\#pzplot\initialize.m
should you want to check it out.