Re: [Gnuplot-bugs] Possible gnuplot bug (Corrected email)

[Hans-Bernhard B. <br...@ph...>]

Zoran Mikic wrote:

>   The attached text file, "bad_contour" has data for a single contour.  
> I have extracted it from one of my contour plots after much debugging.  
> (It was generated with gnuplot.)

This is not technically a bug --- it's a documented feature.  Not a very 
useful one in the case at hand, but that's a different story.

What happens is that this "single contour" of yours is actually three
datablocks (because there are two blank lines in the middle of it). 
What's worse the three datablocks are, by some random chance, of the 
same size.  This means that interpreted as a 3D file, this has "grid 
format", and that unconditionally has gnuplot draw it not as a simple 
curve, but as a surface.  It's the cross-isolines of that surface that
so disturb your plot.

To see this more clearly, try the following command:

splot 'bad_contour' every :::0::0 w l, \
       '' ev :::1::1 w l, '' ev :::2::2 w l

This plots each of the three partial contour curves on its own, in a 
different linetype.

> You can see that the contour folds over itself at every segment.  The 
> connectivity of points in the (three) segments of this contour is not 
> treated correctly.

The connectivity actually present in your file *is* being treated 
correctly.  gnuplot just finds more connectivity than you intended there 
to be, and treats that, too.

To avoid cases like this, you'll have to add an extra point to one of 
the partial contours, or get rid of the breaks (the blank lines) altogether.