A long time ago I made an HP spectrum analyzer display a TV picture. Couldn't hear the sound but it was a fairly decent black and white picture (well green and white). We also used to make oscilloscopes display patterns. I thought I'd try it with the Arduino. I only used programming and some passive components.
Here's three I got going using the Arduino - square, diamond, and circle
I got that scope back in 1986, and I paid $461 for it. A Rigol DS1052 costs about $330. However, this scope is pure analog - no "funny business" going on inside that baby! But ... I'll probably buckle and get a new digital scope, I mean, the screen colors are so pretty ...
Here's a hexagon - Looks better in person because my camera effects the image.
I have 8080 assembler source for a version of the original Space Wars vector graphics game from an old edition of Byte, for display on a scope.
The spacecraft and projectiles are subject to the gravity of the star at the centre of the screen.
I actually implemented it in spare time on an 8080 system I worked on in the early 80s, also implementing a "boss" switch to replace the display with a more normal-looking trace for when management was around.