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From: =?ISO-8859-1?Q?Hans-Bernhard_Br=F6ker?= <HBBroeker@t-online.de>
Newsgroups: comp.graphics.apps.gnuplot
Subject: Re: FIT A DATAFILE THROUGH ANOTHER DATAFILE
Date: Mon, 11 Jun 2012 21:28:54 +0200
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On 11.06.2012 15:27, Roberto Guerra IDV Novi wrote:

> I have a curve in a datafile that is supposed to be the weighted sum
> of other two curves. I wish to find the coefficients for such sum so
> that curve1=curve2*a+curve3*b.
> I tried with the following fit:
>
>   fit 0.0 "<  paste curve1.dat curve2.dat curve3.dat" u 1:($2-$4*a-$6*b)
> via a,b

That cannot work, because gnuplot fits functions, not data.

What that means is that the parameters to be fitted have to appear in 
the _function_ you specify, not in the data's 'using' specification. 
The deeper reason is that gnuplot will not re-read all your data for 
every iteration of the fit.  So the data 'fit' actually sees are built 
with the initial values of 'a' and 'b', and never change after that.  So 
you're trying to 'fit' a constant to a constant.  No wonder that there's 
no result.

A fit that just might work would be:

    fit a*x+b*y \
      "< paste curve1.dat curve2.dat curve3.dat" \
       u 4:6:2:(1.0) \
      via a, b