I'm here to propose adding ZeGrapher to the list, it's a free and open source software for plotting mathematical functions and sequences on the plane, this software was created with the idea that it must be as easy to use as possible, suitable for high school students.
License: Open Source, GNU GPLv3
Description: Plotting mathematical functions and sequences on the plane.
It's developed in C++ and uses Qt framework, the software doesn't use any Registry key and has no config file. So all this would make it more easy to port.