[Gnuplot-info] fit function weirdness

[peterchun <pet...@ma...>]

Hi,
I am having some problems with the fit of a straight line.

I want to get a line of best fit for the 3rd column.
It really should give 
for 
f(x)=m*x+c
m=0
c=293255

but I get 
m=close to 0
c=1

I do not understand how I get a line of best fit that does not fit
What am I doing wrong?

Thanks
Peter


data:
26/08/08 293449 293255
10/09/08 296787 293255
12/09/08 296787 293255
15/09/08 296787 293255
17/09/08 296787 293255
18/09/08 296787 293255
19/09/08 296787 293255
20/09/08 296787 293255
21/09/08 296787 293255
22/09/08 296787 293255
23/09/08 296787 293255
24/09/08 296787 293255
25/09/08 296787 293255
26/09/08 296787 293255
27/09/08 296787 293255
28/09/08 296787 293255
29/09/08 296787 293255
30/09/08 296787 293255
01/10/08 296787 293255
02/10/08 296787 293255
03/10/08 296787 293255
04/10/08 296787 293255
05/10/08 296787 293255
06/10/08 296787 293255
07/10/08 296787 293255
08/10/08 296787 293255
09/10/08 296787 293255
10/10/08 296787 293255
11/10/08 296787 293255
12/10/08 296787 293255
13/10/08 296787 293255
14/10/08 296787 293255
15/10/08 296787 293255
16/10/08 296787 293255
17/10/08 296787 293255
18/10/08 296787 293255
19/10/08 296787 293255
20/10/08 296787 293255
21/10/08 296787 293255
22/10/08 296787 293255
23/10/08 296787 293255
24/10/08 296787 293255
25/10/08 296787 293255
26/10/08 296787 293255
27/10/08 296787 293255
28/10/08 296787 293255
29/10/08 296787 293255



gnuplot commands:
dataset="data"
set xdata time
set timefmt "%d/%m/%y"
set style data boxes
set style fill solid 1 noborder
set xrange [ "29/07/08":"29/01/09" ]
set title "data"
set xlabel "date"
set ylabel "whatever"
set grid
f(x)=m*x + c
fit [ "29/09/08":"28/10/08" ] f(x) dataset using 1:3 via m,c
plot dataset using 1:2 title "line1", dataset using 1:3 title "line2" , f(x)
title "Trendline" lw 3



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