Fityk is a program for nonlinear fitting of analytical functions (especially peak-shaped) to data. Fityk is nonlinear curve-fitting and data analysis software. It is specialized in fitting peaks (bell-shaped functions like Gaussian, Loretzian, Pearson 7, Voigt) to experimental data.

Fityk is reportedly used in crystallography, chromatography, photoluminescence and photoelectron spectroscopy, infrared and Raman spectroscopy, to name but a few.

Fityk knows about common peak-shaped functions (Gaussian, Lorentzian, Voigt, Pearson VII, bifurcated Gaussian, EMG, Doniach-Sunjic, etc.) and polynomials. Fityk also supports user-defined functions.
Fityk offers intuitive graphical interface (and also command line interface), variouse optimization methods (standard Marquardt least-square algorithm, Genetic Algorithms, Nelder-Mead simplex), equality constraints, automation of common tasks with scripts

The license of this software is Free, you can free download and free use this cad software.