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(gnuplot.info)control variables

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 The default epsilon limit (1e-5) may be changed by declaring a value for
       FIT_LIMIT
 When the sum of squared residuals changes between two iteration steps by
 a factor less than this number (epsilon), the fit is considered to have
 'converged'.

 The maximum number of iterations may be limited by declaring a value for
       FIT_MAXITER
 A value of 0 (or not defining it at all)  means that there is no limit.

 If you need even more control about the algorithm, and know the
 Marquardt-Levenberg algorithm well, there are some more variables to
 influence it. The startup value of `lambda` is normally calculated
 automatically from the ML-matrix, but if you want to, you may provide
 your own one with
       FIT_START_LAMBDA
 Specifying FIT_START_LAMBDA as zero or less will re-enable the automatic
 selection. The variable
       FIT_LAMBDA_FACTOR
 gives the factor by which `lambda` is increased or decreased whenever
 the chi-squared target function increased or decreased significantly.
 Setting FIT_LAMBDA_FACTOR to zero re-enables the default factor of
 10.0.

 Oher variables with the FIT_ prefix may be added to `fit` (Note: fit ), so
 it is safer not to use that prefix for user-defined variables.

 The variables FIT_SKIP and FIT_INDEX were used by earlier releases of
 `gnuplot` (Note: gnuplot ) with a 'fit' patch called gnufit and are no
 longer available. The datafile `every` (Note: every ) modifier provides the
 functionality of FIT_SKIP. FIT_INDEX was used for multi-branch fitting, but
 multi-branch fitting of one independent variable is now done as a pseudo-3D
 fit in which the second independent variable and `using` (Note: using ) are
 used to specify the branch. See `fit multi-branch` (Note: multi-branch ).