Free Code Graphics Toolkits and Libraries
"Allegro - a set of basic windowing and 2D graphics functions, programmed in C, and released under the zLib license, a non-copyleft free software license. Used in Factorio (according to user Terkala on Reddit)
Maverick - a virtual reality toolkit "designed to support 3D virtual environments, and interaction with those environments. It uses Mesa or OpenGL to perform low-level rendering, but includes a lot of stuff on top of this to render different kinds of objects, to manage environments and provide support for 3D interaction."
OpenGL (Open Graphics Library), WebGL (Web Graphics Library) etc - cross-platform, cross-language graphics API sets developed by the Kronos Working Group
Visualization Toolkit - 3D graphics, and image processing "