For complicated diagrams, I use xfig. But that has a steep learning curve and is overkill for simple figures. When I saw iPaint, I had visions of the antique MacDraw and MacPaint. They were simple to use and quite intuitive.

I downloaded the app and when I launched it, the interface looked promisingly uncluttered and somewhat MacDraw/MacPaint-like. I selected the line-drawing tool and attempted to draw a short line. The app bombed.

Perhaps there is a fix out there or some trick to make it work, but I'm not inclined to spend any time on it. If an app doesn't do the simple stuff right out of the box, I'll just delete it. Which is precisely what I just did to iPaint. Sorry, Bonzo, no banana!