I wiped the firmware from my AliExpress plotter and decided it would be more fun to write it myself. The root directory you're in right now includes a C++ GUI (made with ImGui) for controlling the plotter with a primitive language, loading SVGs, filling shapes (with my own monotone polygon decomposition), multiple colors and more. The actual firmware is in plotter, which runs on an Arduino with PlatformIO (currently Uno/Due but this is trivial to change). It works about as well as a $100 plotter should:
Here's an outdated screenshot of the UI:
