Re: fit ignores yrange

[sfeam <sf...@us...>]

On Saturday, 06 June 2015 08:48:38 AM pl...@pi... wrote:
> 
> I don't understand where z comes into the discussion in x,y data. :?
> 

Digging into the history of "fit" documentation, here are extracts from
gnuplot.doc for earlier gnuplot versions. (irrelevant lines trimmed)

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Version 4.2.5 (Feb 2009)
 Syntax:
       fit {[xrange] {[yrange]}} <function> '<datafile>'

 Ranges may be specified to temporarily limit the data which is to be fitted;
 any out-of-range data points are ignored. The syntax is
       [{dummy_variable=}{<min>}{:<max>}],
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Version 4.4.0 (Feb 2010)
 The `fit` command can fit a user-supplied expression to a set of data points
 (x,z) or (x,y,z), using an implementation of the nonlinear least-squares
 (NLLS) Marquardt-Levenberg algorithm.

 Syntax:
       fit {<ranges>} <expression>

 Ranges may be specified to temporarily limit the data which is to be fitted;
 any out-of-range data points are ignored. The syntax is
       [{dummy_variable=}{<min>}{:<max>}],

 The default data formats for fitting functions with a single
 independent variable, z=f(x), are z or x:z.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

So the change from treating the function range as "y" to treating it as "z" 
happened in the 4.3 development period and first appeared in the 4.4 release.

I have confirmed that the 4.4.0 executable applied "set zrange"
rather than "set yrange" as a filter on data read by 
    fix f(x) 'data'

So really this discussion is about 5 years too late.
We do try to maintain backwards compatibility where possible,
but ...
 
Given the ambiguity of whether the second pair of inline brackets
in a fit command refers to y or z, isn't it better not to allow this at all,
or issue a warning:
   gnuplot> fit [][ymin:ymax] f(x) 'data' via a,b
   Warning:  bracketed range on "y" is ignored

Note that the documentation has always [incorrectly!] implied that it would
work to say
   gnuplot> fit [][z=ymin:ymax] f(x) 'data' via a,b

Maybe we should fix this and *require* the z= form of the command?
   gnuplot> fit [][ymin:ymax] f(x) 'data' via a,b
   Error:   Second bracketed range in a 1-parameter fit must begin with z=


			Ethan

> >> It's a rather bad regression against previous versions, and one that's
> >> possibly hard to spot for some people. It'd be great if the next release
> >> contained a fix.
> >>
> >> Many thanks in advance, and of course to all developers!
> >>
> >> Best regards
> >>
> >>     Karl
> >>
> >>
> 
> setting xrange, either explicitly or via a range parameter to fit 
> command, works as expected.
> 
>   m=c=0.1;fit [1975:2000] lin(x) datafile u 1:2 via m,c;
> 
> since x is by convention the independent variable and is REQUIRED to 
> have negligible  error  and negligible non linear variability to 
> regression to give an accurate estimation of the supposed linear 
> relationship, this seems appropriate.
> 
> I don't understand where z comes into the discussion in x,y data. :?
> 
> Trying to set the range of the fitting process by defining a range on 
> the dependent variable suggests it may be being applied correctly. I can 
> understand that the algorithm doing the fitting is based on the 
> assumption that x is the independent variable and that is it xrange that 
> defines any subset used for a particular fit.
> 
> Maybe this needs to be stated explicitly in the documentation.
> 
> Peter.
> 
> 
> 
> 
> 
> 
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