a) you are using 'set xdata time'. so the origin of the
coordinate system is ("01/01/2000" , 0).
that means that the point where 'c' is calculated (point of
origin) is far off the point where your data are:
appr. +2.8e+8 seconds.
this can not work.
for the function
f(x)=m*(x-2.8e+08) + c
the fit behaves a little bit better when setting 'c' to a value
near enough to 293255.
but in the end it will terminate with the message
Singular matrix in Invert_RtR
maybe it's because the fit is working at the limit of
computational accuracy? i'm not sure about that...
b) for data distributed on a horizontal line you should
use an appropriate function, e.g.
f(x)=a
and the fit will converge after 4 steps.
> 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
> ...
> 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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