info-gnuplot-digest V1 #2116

info-gnuplot-digest V1 #2116

Post by gavin buxt » Tue, 29 Jul 2003 23:56:25


Sometimes the initial 'guess' of your parameters has
to be reasonably close to the right answer in order
for the fit to converge properly. Try this...

b = -0.08
a = 400
f(x) = a*exp(b*x)
fit f(x) 'temp.dat' via a,b
plot 'temp.dat', f(x)

where your data is in the file 'temp.dat' and the
resultant parameter values are

a               = 253.968
b               = -0.0581577

Hope this helps,


> Hi All,

> I'm trying to use the "fit" function on some data
> that I collected in a
> school experiment, but "fit" keeps on dying and
> giving the error "undefined
> value during function evaluation". I am to fit data
> onto the following data:
> 20 78\n25  61\n30  45\n35  34\n40  24\n45  18\n50  13\n55
> 10
> 60 8\n65   6\n70   4\n75   3\n80   2
> Using the function to be fitted as:
> f(x) = a * exp(-1 * b * x) ; Where a, b are the
> constants fit is supposed to
> give me

> I am using gnuplot v. 3.7 patchlevel 1 on a WinXP
> box. After fit dies, I
> used the command "show vars" to see what a and b
> during fitting and they
> were:
> pi = 3.14159265358979
> a = 3.87069662567025e+40
> b = 8.84029163816757e+42

> Is fit dying because a and b are overflowing? How do
> I get a exponential
> decay model out of my data set? I really need the
> values for my report, you
> see.

> I'm not a member of the mailing list, so send the
> suggestions to

> Thanks in advance.

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1. info-gnuplot-digest V1 #2094


You can control the size of the plotting using xrange
and yrange, which gives you the limits, or bounds, of
your plot. And if you want them to look the same then
you can use set xtics to put the tics where you want,
and also whatever you want. For example....

set xrange[0:10]
set yrange[0:10]
set xtics ("low" 0, "medium" 5, "high" 10)

Set size only changes the size of the graph relative
to the paper, for example if you want to try and make
it look square or something. Hope this helps.....


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