The lines in the last photo in reply #6 are because you're drawing the radius, not plotting points on the circumference.

Forget sine and cosine (you'll either underplot or overplot at some point unless you vary your angular increment).

Forget pi - you're not interested in the area or the length of the circumference.

(sorry don't have any drawing tools to hand)

Imagine you're plotting the circumference of a circle about the origin (0, 0) - we can simply offset this later to whatever part of the screen we want to plot to.

The radius is "r".

r^{2} = x^{2} + y^{2}

so, solving for y,

y = sqrt (r^{2} - x^{2}), and importantly, remember this has two roots, not just the positive one "sqrt" returns.

So now write a "for" loop, from x = 0 to x = x * (sqrt(2) / 2), and plug your values for "x" into the equation for "y".

Then simply "plot(x,y);" where "plot" writes a single point.

This will generate one octant of the circle.

The rest of the circle comes by adding appropriate offsets (remember the observation about "sqrt") and transposing x and y.

Bonus marks for eliminating the "sqrt" operation.

BTW, this isn't a hardware problem.

[edit]Almost forgot - don't forget most graphics devices put the origin at the top left of the screen, and that y increases downwards[/edit]