o_client(3));
	context.lineTo(x_to_client(-3), y_to_client(-3));

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

	context.fillStyle="#222222";
	context.fillText("-π/2", x_to_client(0) - 34, y_to_client(-PI/2) - 4);
	context.fillText("π/2", x_to_client(0) - 29, y_to_client(PI/2) - 4);
}

function draw_canvas() {
	canvas = document.getElementById("canvas");
	var context = canvas.getContext("2d");

	// background
	context.fillStyle="#eeeeee";
	context.fillRect(0, 0, w, h);

	// translati