Draw a circle.
    :param radius: radius in pixels
    :param cairo_color: normalized rgb color

def draw_circle(ctx, x, y, radius, cairo_color):
    """
    Draw a circle.
    :param radius: radius in pixels
    :param cairo_color: normalized rgb color
    """
    ctx.new_path()
    ctx.set_source_rgb(cairo_color.red, cairo_color.green, cairo_color.blue)
    ctx.arc(x, y, radius, 0, 2 * pi)
    ctx.fill()