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Re: Proposal for Matlab style errorbars

From: Ben Abbott
Subject: Re: Proposal for Matlab style errorbars
Date: Thu, 29 Apr 2010 20:49:06 -0400

On Apr 29, 2010, at 3:37 PM, David Bateman wrote:

> Ben Abbott wrote:
>> I've modified __errplot__.m to produce Matlab style errorbars, and would 
>> like some feedback before pushing.
>> I'm not sure how the box style error bars (#) are supposed to look. For my 
>> setup they look identical to the normal ones (i.e. "#~>" looks like "~>"). 
>> Rather than duplicate the same "look", I made the box styles look like 
>> "boxes".
>> I haven't pushed this yet, but did add some demos to errorbar.m. I recommend 
>> pulling that change to make testing/demoing this changeset easier.
>> Of course, as an added bonus, this implementation works with the fltk 
>> backend.
>> Ben
>> p.s. I hope no one minds that I cc'd everyone involved in the discussion on 
>> the bug tracker. 
> As far as I can see the changes to __errplot__ itself are cosmetic.. The big 
> change is in the updata_data subfunction and with the little testing i've 
> done it seems ok. I have two points however. If we make this change to would 
> be a good idea to try and pick the same parameters using in the errorbar 
> lines in the matlab version of this. I don't have matlab. In particular is 
> the value of  dx,  etc the same as used in matlab? It would also make sense 
> to remove the ldata, udata, xldata and xudata from the line objects of 
> Octave, and from __go_draw_axes__ as they will no longer be used after this 
> patch. This values will only be in the hggroup itself
> Cheers
> David

I've verified the width of the error-bar by ...

        xmax = 100;
        x = [0, xmax];
        y = [0, 0];
        dx = diff(x)/100;
        err = [1 1]*dx;
        errorbar (x, y, err, '-sb')
        axis equal
        axis ([-2 2 -2 2]*dx)

Independent of the value for xmax, the resulting errorbar fits into a perfect 
square. I think this confirms that dx = (max(x)-min(x))/100;

This matches what I saw in the figured linked to below.


I'll remove the FIXME comment regarding the errorbar width.

It occurred to me today that semilogxerr, semilogyerr, and  loglogerr each call 
__errplot__ as well. I'll modify dx so that it works correctly with 

Once I've pushed the change to __errplot__, I'll take care of 
__go_draw_axes__.m and graphics.h.in.


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