Incidentally, I don't know if it's common knowledge or not, but if you do this:
i = 0;
do
i++
plot_pixel(sin(i), cos(i));
loop
It draws a circle. Although it works better if you increment i by very small values, it's all good. I learned that a long long time ago when I was playing with QBasic graphics. By plotting SIN and COS against each other I used to draw spirals and waves and circles and stuff.