Scilab is one of the two major open-source alternatives to MATLAB, the other
one being GNU Octave. Scilab is similar enough to MATLAB that some book
authors (who use it) argue that it is easy to transfer skills between the two
systems. Scilab however puts less emphasis on (bidirectional) syntactic
compatibility with MATLAB than Octave does.
QChartist can benefit of the power of the Scilab API through C++ dll calls.
Scilab is used in QChartist for example to draw a conic from five points.
See the source file c:\qchartist\includes\scilab.cpp to understand how it
works. You can find tutorials for Scilab in c:\qchartist\docs\scilab\