Re: Gnuplot and Octave.

[Daniel J S. <dan...@ie...>]

Hans-Bernhard Broeker wrote:

>On Fri, 20 Feb 2004, Ole Jacob Hagen wrote:
>
>  
>
>>Gnuplot has visualisation properties, which could be interpreted, and
>>used by Octave by our package. But, there is always a but. We need to
>>make duplicate copies of datasets, which are plotted in Gnuplot, since
>>Gnuplot doesnt send it's dataset of visualisation back.
>>    
>>
>
>I don't think I see what that "dataset of visualization" you're talking
>about is.
>
>Anyway: is this about 3D or 2D plots?  Or colour maps?
>
>  
>
>>This is the curve/surface data, such as X, Y and Z.
>>    
>>
>
>How would these differ from the data sent from octave to gnuplot?
>

Probably most on this list are familiar with Matlab's handle graphics 
objects, but just in case I'll summarize.  Matlab extends the concept of 
matrices to the plot realm by associating elements of a plot to an 
elements in a vector.  Think of these elements as "handles".  Using 
"get" will get the vector of handles associated with a particular 
figure.  For example, here is a short segment of code to change the line 
width of a plot:

  H = get(get(gcf,'Children'),'Children');
  set(H(1),'LineWidth',2.0);

You'll notice to use of "get" twice.  The reason is that there is a 
hierarchy of plot elements.  For example, an axis will be an element, 
and then under that will be other elements for the axis like text. 
 "get" alone without assignment will display all the information about a 
handle object such as the actual text, the font type, the size of the 
font, etc.  The sub-objects are called children.

Naturally, this requires a storing of all information about the plot 
elements.  Or, it requires some way of inquiring such information from 
Gnuplot, if Gnuplot keeps a record of such things.  A plot line is also 
an object and part of its characteristics is all the data points 
associated with it.  So, this is what the original question is about. 
 Is it possible to get back the (x,y,z) information that Petr is 
referring to from Gnuplot.

A few things:

1)  This would mean that somehow information about plot data would need 
to be sent back to Octave through the pipe so that Octave could 
interpret it and use it in a handle graphics fashion.  I'm not sure if 
Gnuplot is set up to do such a thing.

2)  It would then seem that to do this would require the strategy that, 
I think it was, Hans has mentioned about each Gnuplot plot storing all 
information so that it can be retrieved.  (I assume same goes for all 
other elements like labels, etc.)  From Gnuplot's perspective, this was 
for allowing the mouse to zoom on not only the most recent plot, but all 
plots in X11.  But here would be an example of another use.

3)  I don't know what Ole has in mind, but I assume that this concept 
has to be extended to all other graph objects, i.e., labels and so on. 
 That is, either Gnuplot is the one to keep track of all the retrievable 
information, or Octave is the one.

You may be wondering, how does one know what the hierarchy is Matlab? 
 That is, how did I know to use "get" twice in the above script example 
and then select element H(1) from the array of handle objects?  Well, 
one has to simply display the information, following the hierarchy of 
children down to the object of interest.  It isn't the most apparent 
logic as to how things are grouped, but perhaps once you get used to it, 
things become easier.

However, I'm going to give my experience about this, and keep in mind 
that this is opinion.  (Repeat OPINION!)  I was exposed to Matlab before 
Octave so the handle object approach is what I first used to customize 
plots.  I never found it easy to use; quite arduous actually.  The 
searching through children of children of children part is what gets to 
one after while.  Switching over to Octave, I found that using "gset" 
and the Gnuplot customizing commands a bit easier to use.  The handle 
graphics approach is to create the graphics objects and then go back and 
modify them (going through the handle hierarchy in the process).  The 
Gnuplot approach is to set the object property at the same time the 
object is created, and be done with it.  In Gnuplot, setting a default 
property is often very easy as well.

I could say more here, but I will leave it open for further comment from 
others.

Dan