As I needed an OVAL drawing routine for the paint cart I'm working on, I was surprised to see how small of code it was required in order to do such an effective one !
Here is the original C source code by roger dahl found in stackoverflow.com :
Here is my code for Pico-8:
As you can see it's a real plotting monster, but it DOES work, and I checked, it does not draw over any pixels it previously drew, so it's going about as fast as it can I think.