func clear(s draw.Image) {
	b := s.Bounds()
	for i := b.Min.X; i < b.Max.X; i++ {
		for j := b.Min.Y; j < b.Max.Y; j++ {
			s.Set(i, j, black)
		}
	}
}

// http://en.wikipedia.org/wiki/Midpoint_circle_algorithm
func drawCircle(img draw.Image, cx int, cy int, r int, c image.RGBAColor, fc image.RGBAColor) {
	x := r
	y := 0
	error := -r
	for x >= y {
		var t int
		t = cy + y
		if fc.A != uint8(0) {
			drawHorizontalLine(img, cx-x, cx+x, t, fc)
		}
		if c.A != uint8(0) {
			set(img, cx+x, t, c)
			set(img, cx-x, t, c)
		}
		t = cy - y
		if fc.A != uint8(0) {
			drawHorizontalLine(img, cx-x, cx+x, t, fc)
		}
		if c.A != uint8(0) {
			set(img, cx+x, t, c)
			set(img, cx-x, t, c)
		}
		t = cy + x
		if fc.A != uint8(0) {
			drawHorizontalLine(img, cx-y, cx+y, t, fc)
		}
		if c.A != uint8(0) {
			set(img, cx+y, t, c)
			set(img, cx-y, t, c)
		}
		t = cy - x
		if fc.A != uint8(0) {
			drawHorizontalLine(img, cx-y, cx+y, t, fc)
		}
		if c.A != uint8(0) {
			img.Set(cx+y, t, c)
			img.Set(cx-y, t, c)
		}
		error += y
		y += 1
		error += y
		if error >= 0 {
			x -= 1
			error -= x
			error -= x
		}
	}
}

func plot(s draw.Image, v []Point) {
	clear(s)
	min, max := scale(v)
	b := s.Bounds()
	xscale := float64(b.Max.X-b.Min.X) / (max.x - min.x) * Shrink
	yscale := float64(b.Max.Y-b.Min.Y) / (max.y - min.y) * Shrink
	xoffset := -(min.x * xscale) + 4
	yoffset := -(min.y * yscale) + 4
	for _, p := range v {
		x := int(xscale*p.x + xoffset)
		y := int(yscale*p.y + yoffset)
		if x < 0 || y < 0 {
			fmt.Println(min, max)
			fmt.Println(x, y)
			fmt.Println(p.x, xscale, xoffset)
			fmt.Println(p.y, yscale, yoffset)
		}
		s.Set(x, y, Color)
	}
}

func set(img draw.Image, x, y int, c image.RGBAColor) {
	if x >= 0 && y >= 0 && x < img.Width() && y < img.Height() {
		img.Set(x, y, c)
	}
}