[Gnuplot-info] wrong simple linear fit

[Eckard S. <sp...@hy...>]

Hi@all,

I'm not sure if my fit problem with gnuplot is an old hat. I have built a site
that helps students to get results from physical experiments by the help of
gnuplot. All is well but for a simple linear fit I get wrong parameters.

My data are:
6.03e5  0.4
6.79e5  0.45
7.54e5  0.5
8.30e5  0.55
9.05e5  0.6
9.81e5  0.65
which are stored in a file 'lreg3.dat'.

My gnuplot commands are:
f(x) = a*x+b
fit f(x) 'lreg3.dat' via a,b
plot f(x), 'lreg3.dat'

gnuplot comes up with a totally wrong regression line (i.e. a negative slope).
If one scratches one data pair from list, the result is correct.

I have read in 'help fit' that this misfit can occur if the magnitudes of both
parameters a, b differ too much. This may be the case here. But a simple linear
fit should always work. Or not?

Is it true that the Marquardt-Levenberg algorithm is used for all fits?

Must I provide 'good' starting values for a, b to get a correct result? This
would be very inconvenient here.

Thank you very much for your help/comments.

Regards,
-Eckard