[Gnuplot-info] Fit trouble when I use xdata time (fit isn't so good)

[Guilherme P. <gpo...@gm...>]

Hi!
 
I am a new user of gnuplot and my version is 4.2 patchlevel 6. I'm having a 
trouble with "fit" function when I use xdata time as X axis values. Here my 
script and data set.
 
# watthour.gnu
# =========== < cut here
reset
set terminal png size 800,500 font comic 12 enhanced truecolor
set output "watthour.png"
set xdata time
set timefmt "%Y/%m/%d %H:%M"
set label "Powered by GnuPlot 4.2" at graph 0.87,-1.7 center font "lhandw,10" 
textcolor rgb "#800080"
show label
set key below left vertical title "Variables"
set border 3
set xtics rotate by 45 offset graph -0.075,-0.30 textcolor rgb "#008000" 
nomirror
set ytics nomirror textcolor rgb "#008000" nomirror
set format x "%d%b %Hh"
set format x "%N"
set format y "%6.1f"
set xtics 86400
set ytics 2
set mxtics 12
set mytics 5
f(x)=a + b*x 
fit f(x) "watthour.txt" using 1:4 via a,b
set title "Kilowatts - 2010\n Fit Model: f(t) = a + b.t " font "comic, 12" 
textcolor rgb "#800080"
plot f(x) t"Projection" with lines 2, "watthour.txt" using 1:4 t "KWh" with 
points 3
# =========== < cut here
 

 
# watthour.txt
# =========== < cut here
#YYYY MM DD hh mm U  KWh
2010/02/19 23:36 3 22835.1
2010/02/20 08:05 3 22839.0
2010/02/20 10:32 3 22839.8
2010/02/20 11:32 3 22840.1
2010/02/20 12:49 3 22840.8
2010/02/20 23:28 3 22845.5
2010/02/21 01:40 3 22846.8
2010/02/21 13:45 3 22851.5
2010/02/21 22:09 3 22855.6
2010/02/22 18:06 3 22864.5
2010/02/22 22:14 3 22866.0
# =========== < cut here
 
 
 
I believe that are a problem with "fit" function when I use xdata time, 
because X values are converted to second since 2000/01/01 ( I think! ) and 
this number get big compared to Y values... the algorith to "fit" the data to 
the model has some dificulties with this. The curve not pass near of the 
points, of course, it not pass over them, but should be pass so near because 
it is a adjustment with "least squares"!! My data set file don't have outliers 
to cause the problem that I report here.
 
I tried do some tests with this same data, changing time near year 2000 and 
the fit curv was diferent, but it curve should be the same angular coeficient. 
Is it a bug or I'm commiting a mistake.
 
Thank you very much!
 
__________________________________
Guilherme Poleszuk dos Santos Rosa
São Paulo, Brazil