[Gnuplot-info] How to fit a line in log plot or how to help converge exp better

[<lyn...@fr...>]

Hello!


I found quite a lot of info on fitting exponential functions but could not get
a good result with my FIT_LIMIT and FIT_MAXITER. I wonder if it is possible to
fit  just  a  line  in  a  single log plot? Or how can I use the other fitting
parameters to get a fit that looks right in single log plotting style  of  the
data below. It hasn't got many values but I can't get more.

Two other not so important questions:
Would there be a way to loop this over the other data columns as well?
Is  there  a better way to get the axis labels close to the last tic posistion
to save space?

Any help or hints welcome
Lynx

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#set terminal png font FreeSans 14 crop size 600,400
#set xlabel "Spz"
#set lmargin 20
set xrange [1:12]
set xtics 1,1,11
set label 1 'Spz' at graph 0.97, graph -.05
set yrange [1e-12:9e-8]
set mytics 10
#set ytics 0,1e-,1.1e-5
set label 2 'I[A]' at graph -0.05, graph 0.99
#set ylabel "P[mbar]"  offset screen 0.3,0.8 rotate by 0
#set ylabel "P[mbar]"  offset 4,0 rotate by 0
set key right top  Left reverse
set logscale y
set termoption enhanced
#f(x) = a*b**(x)  
f(x) = a * exp (-x*b) 
FIT_LIMIT = 1e-99
FIT_MAXITER = 0
fit [1:11] f(x) '< sed "s/ 0/ ?0/g" dosis.dat' using 1:3 via a,b
#plot a*b**(x) 
plot '< sed "s/ 0/ ?0/g" dosis.dat' using 1:3 t "C_2: 1" ls 1 with lp,  a * exp (-x*b)   
#, '< sed "s/ 0/  ?0/g" dosis.dat' using 1:4  t "C_2: 2" ls 2 with lp, '< sed "s/ 0/  ?0/g" dosis.dat' using 1:5 t "C_2: 3" ls 3 with lp, '< sed "s/ 0/  ?0/g" dosis.dat' using 1:6 t "C_2: 4" ls 4 with lp

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 dosis.dat 
#Spz diameter/nm I1/A I2/a I3/A I4/A
 1 400 2.6e-8  6.6e-9  1.8e-9  1.8e-10
 2 350 1.8e-8  4.5e-9  1.2e-9  1.3e-10
 3 300 1.3e-8  3.3e-9  8.5e-10 9.1e-11
 4 210 6.1e-9  1.7e-9  4.3e-10 4.9e-11
 5 140 2.6e-9  6.8e-10 1.8e-10 0
 6  95 1.4e-9  4e-10   1e-10   0
 7  52 6.3e-10 1.8e-10 5.2e-11 0
 8  34 2.4e-10 7e-11   0       0
 9  18 9.1e-11 3.3e-11 0       0
10  12 5.2e-11 0       0       0
11   9 3.8e-11 0       0       0