Fityk

Fityk  is an advanced and reliable software solution whose main purpose is to assist users in fitting data to nonlinear functions. It is aimed at experimental data resulted from a variety of domains, like chromatography, spectroscopy, crystallography and many others.
The program is fairly simple to work with, requiring minimal levels of knowledge or experience with similar tools, yet it features a set of rather complex functions.
These require a bit of documentation before being able to work with them properly, as such it is less approachable for novice users, though the more advanced can certainly appreciate its complexity.
The interface of Fityk is quite easy to understand, offering a toolbar for quick access to the most commonly used functions, as well as a menu bar, a main plot and a right-size panel where certain values can be edited.
Fityk provides users with several fitting methods, namely 'Levenberg – Marquardt', 'Nelder - Mead Simplex' and 'Genetic Algorithm', which can be of use in multiple analysis. Users can work in 'Data Range', 'Add Peak' or 'Baseline' mode, according to their needs.
The utility allows users to execute a script, use data point 'Transformations' or adjust the preferred 'Functions', with the possibility of choosing between a wide range of options, such as 'Quadratic', 'Cubic', 'Gaussian', 'Lorentzian', 'EMG', 'Polyline', 'ExpDecay', 'Voigt', 'Spline', 'DoniachSunjic' and many more, while also letting users create their own.
Moreover, Fityk enables users to perform 'Power Diffraction Analysis' using 'X-Ray' or 'Neutron' radiation and various wavelengths ('Cu', 'Ag', 'Mo','Fe', etc). The tool allows users to exclude inactivate points from fitting operations and work with several datasets simultaneously.
To summarize, Fityk is a comprehensive and efficient application that is meant to help users perform nonlinear data fitting using as a basis the results of experiments from numerous fields of scientific activity.