Fityk is A Multi-platform Open-source Data-analysis Package for Scientists

Fityk [fi:tik] is an open-source program for data processing and nonlinear curve fitting, that works for Windows, Linux and macOS.

Usage

It is primarily used:

by scientists who analyse data from powder diffraction, chromatography, photoluminescence and photoelectron spectroscopy, infrared and Raman spectroscopy, and other experimental techniques,
to fit peaks – bell-shaped functions (Gaussian, Lorentzian, Voigt, Pearson VII, bifurcated Gaussian, EMG, Doniach-Sunjic, etc.),

but it is suitable for fitting any curve to 2D (x,y) data.

intuitive graphical interface (and also command line interface),
support for many data file formats, thanks to the xylib library,
dozens of built-in functions and support for user-defined functions,
equality constraints,
fitting systematic errors of the x coordinate of points (for example instrumental zero error or sample displacement correction in powder diffraction),
manual, graphical placement of peaks and auto-placement using peak detection algorithm,
various optimization methods (standard Marquardt least-squares algorithm, Genetic Algorithms, Nelder-Mead simplex),
handling series of datasets,
automation with macros (scripts) and embedded Lua for more complex scripting
the accuracy of nonlinear regression verified with reference datasets from NIST,
an add-on for powder diffraction data (Pawley refinement)
modular architecture,
open source license (GPLv2+).