[Gnuplot-info] Gnuplot "fit" command

[<ra...@be...>]

I was looking for a good, simple program that will let me fit some data to
theoretical models. I decided to have a look at gnuplot and test it by
seeing if it can perform a simple linear fit. With the file attached
(testfile.csv), which fits y=0.4*x, I executed

gnuplot> f(x) = a*x
gnuplot> fit f(x) "testfile.csv" via a

I got the result

Iteration 3
WSSR        : 4.13793           delta(WSSR)/WSSR   : -4.55122e-09
delta(WSSR) : -1.88326e-08      limit for stopping : 1e-05
lambda    : 0.00822598

resultant parameter values

a               = 1.10345

After 3 iterations the fit converged.
final sum of squares of residuals : 4.13793
rel. change during last iteration : -4.55122e-09

degrees of freedom (ndf) : 14
rms of residuals      (stdfit) = sqrt(WSSR/ndf)      : 0.54366
variance of residuals (reduced chisquare) = WSSR/ndf : 0.295567

Final set of parameters            Asymptotic Standard Error
=======================            ==========================

a               = 1.10345          +/- 0.01706      (1.546%)


correlation matrix of the fit parameters:

               a
a               1.000



Either I am missing something, or there is something very wrong with
gnuplot's fit command. The data should fit y=0.4*x exactly. I've tried
setting a to 0.43 before running the fit command and gotten the same
result.

Any help?

- Jonathan