Wikipedia
QtiPlot is a cross-platform computer program for interactive scientific graphing and data analysis. It is similar to the proprietary programs Origin or SigmaPlot, and is being used to substitute these in universities.
QtiPlot can be used to present 2D and 3D data and has various data analysis functions like curve fitting. Plotting of 3D data can be rendered using OpenGL using the Qwt3D libraries.
The program is also extensible to a considerable degree via muParser and Python scripting language, which allows adding the arbitrary user-defined functions with access to graphs, matrices and data tables.
QtiPlot up to version 0.9.8.9 was released under the terms of the GNU General Public License (GPL), but the source code is no longer available starting with version 0.9.9 free software. Compiled binaries up to this version are available for Microsoft Windows, several Linux distributions and Mac OS X. Downloading binaries from the author's website requires purchase of an annual maintenance contract, however anyone is allowed to compile and distribute binaries under the GPL. This is the case for some Linux distribution repositories as well as for Windows.