custom functionfit in libreoffice calc

data.csv

plotfit.gp

set datafile separator ","

set xrange [0:1800]
set yrange [0.14:0.2]

a=0.0633
b=0.005092
c=0.155
f(x)=a*exp(-b*x)+c
fit f(x) 'data.csv' u 1:2 via a,b,c

plot 'data.csv' using 1:2 title 'data'  with lines,\
f(x) title sprintf('%.2f *e^(- %.2f x)+%.2f', a,b,c)

pause -1

plotfit.py

import numpy as np
data=np.loadtxt(open("data.csv", "rb"), delimiter=",", skiprows=1)

from scipy.optimize import curve_fit
def func(x, a, b, c):
    return a * np.exp(-b * x) + c

popt, pcov = curve_fit(func, data[:,0], data[:,1],p0=[0.0633,0.005092,0.155])

import matplotlib.pyplot as plt

plt.plot(data[:,0],data[:,1],label='data')
plt.plot(data[:,0], func(data[:,0], *popt), 'r-', label='fit: a=%5.3f, b=%5.3f, c=%5.3f' % tuple(popt))

plt.legend()

plt.show()