Fityk (program for data processing and nonlinear curve fitting)

Fityk [fi:tik] is a program for data processing and nonlinear curve
fitting.

Primarily used:

- by scientists who analyze 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.

Fityk is distributed under an open source licence (GPLv2+).

Optional dependency: gnuplot (can be used with the CLI)
