Fire
Pieced together from samples from Adafruit's ProtoMatter library and some code I found here, created this fire effect display on a Matrix Portal Starter Kit.
Kept playing with it until I got the randomisation effect that I wanted.
How to install
If you have the aforementioned kit, there's no need to build it yourself. Just download the UF2 firmware, start the bootloader and drop the firmware on the board.