As in graphics in the terminal? The way that works is through the draw(3) kernel device which is a 2d engine with an rpc interface. You load text and bitmaps into the draw device and then issue rendering commands. The kernel terminal device, cons(3) is where the terminal text is written to and cons sends that to to draw. When you start a graphical program, it overwrites the window graphics from cons(3) until the graphical program exits or the window deleted. There is no in band cursor control in plan 9 as it is a graphics oriented OS.
So its not just porting a terminal. It's the entire OS. Of course there is p9p, plan 9 port, which is a port of the core plan 9 user space tools to Unix systems. It does offer a draw server that can be mounted.
A lot of terminals supports at least somewhat similar functionality via Sixel (bitmaps) and ReGIS (vector graphics). It's limited and certainly could be improved a lot, though.
So its not just porting a terminal. It's the entire OS. Of course there is p9p, plan 9 port, which is a port of the core plan 9 user space tools to Unix systems. It does offer a draw server that can be mounted.