Groups > comp.graphics.apps.gnuplot > #3567

Strange thing about logscale after a fit ?

Newsgroups	comp.graphics.apps.gnuplot
Date	2017-03-08 10:07 -0800
Message-ID	<24ea3aba-292c-4afb-a4a8-59685203a23f@googlegroups.com> (permalink)
Subject	Strange thing about logscale after a fit ?
From	Vincent Gouret <vincentgouret44@gmail.com>

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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...

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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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