How to write function to plot?

[John B. Thoo]

Hi, everyone.  I have a question about writing a function for  
plotting some data saved to a file.

I have some data saved to a file "heat_q.mat" for the variables k, x,  
t, u.  Here's a snippet.

%%%%%%%%%% begin snippet %%%%%%%%%%%
# Created by Octave 3.0.5, Fri Apr 24 13:11:38 2009 PDT <address@hidden 
Thoos-Computer.local>
# name: k
# type: scalar
3
# name: x
# type: matrix
# rows: 1
# columns: 21
 0 0.3141592653589793 0.6283185307179586 0.9424777960769379  
1.256637061435917
...
# name: t
# type: range
# base, limit, increment
0 1 0.1
# name: u
# type: matrix
# rows: 21
# columns: 11
 0 0 0 0 0 0 0 0 0 0 0
 0.8090169943749475 0.3323236694342593 0.136510137653631  
0.05607490346365374
...
%%%%%%%%%% end snippet %%%%%%%%%%%

If I load the file and then enter on the command line

> l = 3;
> plot (x, u(:,l), '-ok', x, exp (-k^2*t(l))*sin (k*x), '-b');

then I get a plot.  However, if I use the following function

%%%%%%%%%%%%%%%%%%%%%%%
function plotheat_q (l)

% plot numerical solution and exact solution at time  t0+l*dt;
plot (x, u(:,l), '-ok', x, exp (-k^2*t(l))*sin (k*x), '-b');

endfunction
%%%%%%%%%%%%%%%%%%%%%%%

then I get the following error:

octave-3.0.5:2> load heat_q.mat k x t u
octave-3.0.5:3> plotheat_q (3)
error: `x' undefined near line 4 column 7
error: evaluating argument list element number 1
error: called from `plotheat_q' in file `/Users/jbthoo/Desktop/ 
NumMethPractice/plotheat_q.m'
octave-3.0.5:3>

What am I doing wrongly?

Thanks.

---John.