[Gnuplot-info] Problem with fitting

[Edward H. <ph...@le...>]

Hi,

I have a problem with fitting data to a dataset with uncertainties in 
gnuplot 4.0 under Linux.

The data file I am using is of the form:
x   y   xerror   yerror

I can plot this data complete with error bars with the line:
plot "data.txt" using 1:2:3:4 with xyerrorbars

This works perfectly, what doesn't work is trying to fit a straight line 
too it for which I am trying:
f(x) = a*x + b
a = 1e-23
b = 0
fit f(x) 'data.txt' using 1:2:3:4 via a, b

For some reason the line I get (which is meant to be calculated from 
uncertainties) is far from where it should be.  It runs at y = 
0.00635613x + 0.0041886, which is off the scale of the points that I'm 
dealing with (xrange [0:0.5], yrange [0:4e-24]).

Any ideas what is going wrong?  I've tried numerous 'using' arguments to 
no avail.

Here is my entire script:
clear; reset
f(x) = a*x + b
a = 1e-23
b = 0
fit f(x) 'data.txt' using 1:2:3:4 via a, b
set terminal png font "/mnt/win_c/WINNT/Fonts/verdana.ttf" 11 xffffff 
x000000 x000000 x000000 x000000
set output "mb.png"
set title "Magnetic field strength vs dE"
set xlabel "B (T)"
set ylabel "dE (J)"
set key off
plot "data.txt" using 1:2:3:4 with xyerrorbars
set xrange [0:0.5]; replot
set yrange [0:4e-24]; replot
replot f(x)