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Groups > comp.graphics.apps.gnuplot > #3569

Re: Strange thing about logscale after a fit ?

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Le mercredi 8 mars 2017 20:01:05 UTC+1, Ethan A Merritt a écrit :
> Vincent Gouret wrote:
> 
> > Hello all,
> > 
> > I have a strange result for plotting data in logscale. The file I use
> > contains this:
> > --------------------
> > temps_gauss.txt:
> > 
> > 5	100000	4
> > 6	69444	4
> > 7	51020	4
> > 9	30863	5
> > 10	24999	5
> > 12	17360	6
> > 13	14791	7
> > 16	9764	9
> > 18	7714	11
> > 21	5667	15
> > 25	3998	22
> > 29	2971	31
> > 34	2161	47
> > 40	1561	70
> > 47	1130	110
> > 55	825	157
> > 64	609	232
> > 75	443	361
> > 88	321	552
> > 103	234	822
> > 120	172	1281
> > 141	124	2031
> > 165	90	3148
> > 193	65	4941
> > 227	46	7883
> > 265	33	12483
> > 311	23	19784
> > 364	16	31094
> > 427	11	50103
> > 501	7	80055
> > ----------------------
> > The first colum is the size of a linear problem, the second is the number
> > of random problems of this size and the last is the mean execution time in
> > us. The linear problem is solve using the Gauss algorithm. I plot this
> > result in Gnuplot with: plot 'temps_gauss.txt' u 1:3 w p It work fine. If
> > I want to see the same plot with logscale, I do this: set logscale
> > and when I redo the plot, all goes fine. After that, I do a fit like that:
> > f(x)=alpha*x**3
> > fit f(x) 'temps_gauss.txt' u 1:3 via alpha
> > This works well and give me a value for alpha=6.4e-4, fine. Then I plot
> > the datas and the model, and at this time it does not work: plot
> > 'temps_gauss.txt' u 1:3 w p, f(x) w l This give me the error:
> >  color axis has cb coord of 0; must be above 0 for log scale!
> > If I stop gnuplot, and relaunch it, and type:
> > f(x)=alpha*x**3
> > set logscale
> > alpha=6.4e-4
> > plot 'temps_gauss.txt' u 1:3 w p, f(x) w l
> > then all goes fine ??? What is the problem ? I use gnuplot V5 patchlevel
> > 4, under Windows 8.1 or V5 patchlevel 3 under Lubuntu 16.04.2, the same...
> 
> The error occurs because 
> 
> - the command "set logscale" applies to all axes, not just x and y.  
> - the default range for all axes in the absence of any input is [-10:10]
> - if you actually use an axis in the plot, the program checks that the
>   range is reasonable, which in the case of log scale means 
>   adjusting the default it to be > 0 if necessary.
> - unfortunately (here is the bug) the color axis range is not adjusted
>   for log scale because your plot does not actually use it.  So the 
>   default range is incompatible with log scale and you get an error.
>   I do not know exactly how long this bug has been there, but I will file
>   a bug tracker item so that it gets fixed in the next gnuplot release.
> 
> To avoid the error, any one of the following is sufficient
> 
> - Only set log scale for x and y:
>     set log xy
> - Clear log scale for the color axis:
>     unset log cb
> - Set the range of the color axis to be >0
>     set cbrange [1:*]
> 
> 
> 	Ethan

Thanks a lot Ethan, the only thing to to is to add tyour command:
unset log cb
and the problem goes away. Your remark about default range is not releavant.

Now it works :-)

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Thread

Strange thing about logscale after a fit ? Vincent Gouret <vincentgouret44@gmail.com> - 2017-03-08 10:07 -0800
  Re: Strange thing about logscale after a fit ? Ethan A Merritt <sfeam@users.sourceforge.net> - 2017-03-08 11:01 -0800
    Re: Strange thing about logscale after a fit ? Vincent Gouret <vincentgouret44@gmail.com> - 2017-03-08 11:16 -0800

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