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(gnuplot.info)multi-branch

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 In multi-branch fitting, multiple data sets can be simultaneously fit with
 functions of one independent variable having common parameters by minimizing
 the total WSSR.  The function and parameters (branch) for each data set are
 selected by using a 'pseudo-variable', e.g., either the dataline number (a
 'column' index of -1) or the datafile index (-2), as the second independent
 variable.

 Example:  Given two exponential decays of the form, z=f(x), each describing
 a different data set but having a common decay time, estimate the values of
 the parameters.  If the datafile has the format x:z:s, then
      f(x,y) = (y==0) ? a*exp(-x/tau) : b*exp(-x/tau)
      fit f(x,y) 'datafile' using  1:-1:2:3  via a, b, tau

 For a more complicated example, see the file "hexa.fnc" used by the
 "fit.dem" demo.

 Appropriate weighting may be required since unit weights may cause one
 branch to predominate if there is a difference in the scale of the dependent
 variable.  Fitting each branch separately, using the multi-branch solution
 as initial values, may give an indication as to the relative effect of each
 branch on the joint solution.