(-PI/2), y_to_client(3));
	context.lineTo(x_to_client(-PI/2), y_to_client(-3));

	context.stroke();
	context.closePath();
	context.setLineDash([]);

	context.fillStyle="#222222";
	context.fillText("π/2", x_to_client(PI/2) + 6, y_to_client(0) - 6);
}

function draw_atan() {
	// client
	const w = 640.0;
	const h = 480.0;

	// visible region
	const x_min = -4.5;
	const y_min = -3.0;
	const x_max = 4.5;
	const y_max = 3.0;
	const client_x_center = w*(0-x_min)/(x_max-x_min);
	const client_x_step = (w/(x_max-x_m