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Getting started with matlab guide, hit a stumbling block. Got it as simple as I can as a toy gui to illustrate my problem. A gui (named asas) has a pushbutton and an axis. The callback of the button reads 

axesHandle= findobj(gcf,'Tag','axes1');
x=rand(randi(10+20,1),4);
plot(axesHandle, x)

There's no other code written by me (guide wrote it).
The 1st time I push the button, everything is fine: the plot gets done. The 2nd time around, I get an error, from the console:

Error using plot
Vectors must be the same lengths.

Error in asas>pushbutton1_Callback (line 83)
plot(axesHandle, x)

Error in gui_mainfcn (line 96)
        feval(varargin{:});
etc...

I want to plot the new data x, replacing the old one.
It looks like matlab is not replacing the data to plot, but somehow trying to append to the plot?

I have searched, but haven't found anything that applies.

Ole Thomsen Buus
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pedro silva
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  • Does this work, i.e. just this code x=rand(randi(10+20,1),4); plot(x)? – Dan Jul 09 '12 at 14:31
  • Thanx Dan for a quick reply. yes, it works! I thought that I always had to specify where to plot -- goes to show my ignorance, i guess. Any pointers to why my original code was wrong (Dan or anyone else), please? – pedro silva Jul 09 '12 at 14:36
  • I would guess that the way you had it before wasn't clearing the graph. It is probably better to keep it similar to how you had it before but just manually clear the graph before plotting again but I'm not 100% about all that. – Dan Jul 09 '12 at 14:47
  • In terms of replacing the old plot, could you simply implement `clf` upon the button press, so it'll always try to plot on a clean plot. – gkiar Jul 09 '12 at 17:08

1 Answers1

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The explanation is not straightforward - and certainly not if you are new with MATLAB and its handle graphics subsystem.

Your code as it is, line by line:

axesHandle= findobj(gcf,'Tag','axes1');
x=rand(randi(10+20,1),4);
plot(axesHandle, x);

The first line attempts to locate in the current figure (gcf, "get current figure") any child object with the property 'Tag' set to the string 'axes1'. I guess you are aware of this? The second line of course generates some random data to plot. The third line plots the data in x.

But after the plot-call the property 'Tag' is actually reset to '' (the empty string), which in turn makes findobj fail in any subsequent searches for the axes-handle. The variable axesHandle with therefore NOT contain an actual handle but instead the empty matrix []. This will make plot default to another mode an interpret the empty matrix as data for the x-axes (the first argument to plot). This expectedly results in the error you receive:

...
Error using plot Vectors must be the same lengths.
...

The solution by Dan in the comment above is a workaround, but there is good sense in telling plot where to plot - especially in GUIs.

You can instead add a fourth line:

set(axesHandle,'Tag','axes1');

This will set property 'Tag' back to 'axes1' and any subsequent clicks on the button should now also work. And you can add more than one axes-objects now. If that is what you want to.

Ole Thomsen Buus
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  • Thanx again to all who replied, especially to Ole for beautifying my question, and providing a long and illuminating answer. I would certainly have upped your answer, were I "reputable" enough (although I might have also changed "it's" to "its" in the first line...). I had absolutely no idea that after the plot-call the property 'Tag' was reset to the empty string. I have followed your "4th line" solution, and it worked. – pedro silva Jul 09 '12 at 21:19
  • Now you should be able to vote up :P I am happy it worked. Please also accept the answer if it solved your problem. – Ole Thomsen Buus Jul 09 '12 at 21:50