[Gnuplot-info] Fitting and inverse function

[Theo <th...@gm...>]

Hi all,

I have a function e.g. like
f(x,T1,T2,T3,R1,R2,R3) = R1*exp(-x/T1) + R2*exp(-x/T2) + R3*exp(-x/T3)

When using

fit f(x,T1,T2,T3,R1,R2,R3) 'FileName.dat' using 1:2 via T1,T2,T3,R1,R2,R3

I nicely get the best fitting constants T1,T2,T3,R1,R2,R3

Now, I now would like to know what is x if y = f(x) = 0.5 and place it 
as a label. The problem is that the above function can not be simply 
rewritten as inverse function x = g(y)

At least as an approximation, how could I extract x?
Can I use the fitting capability of gnuplot to do that? I don't see how 
to do it. Any ideas?

Thanks,
Theo.