[Newbies] How to draw a single pixel with permanence?

I'm trying to go back to beginnings with my own ManelBrot morph because I can quite grok how the one I have works.

Mostly because I still dont grok morphs.

Example: I'm trying to plot points on a morph using the drawOn message below:

drawOn: aCanvas "Draws the receiver on the given canvas"

super drawOn: aCanvas. aCanvas form colorAt: 30@30 put: Color black.

BUT, with or without the evocation of super drawOn, I can't get the point to stay around. It only draws during a drag. What is the pattern for this. I've looked at much code and many tutorials but they all seem to gloss over this little detail, or at least my eye skips it every time I look at all the examples I can find.

Lawson