Nothing special. Microcontroller requests a new point, computer sends a new point. If there are no new points, computer sends an end transmission code.
while stop flag not set
If buffer is empty
if received end transmission
set stop flag
draw all points in buffer
One More Thing
The Z height needs to be adjustable. Get a couple buttons to adjust the drawing height, so the pen touches, not misses or jams into, the paper.
With that done, its picture time!