ion init_both_canvas(){
	var canvas = document.getElementById("both_canvas");
	canvas.addEventListener('pointerdown', function(e){
		canvas_rect = canvas.getBoundingClientRect();
		const xy = [e.clientX - canvas_rect.left, e.clientY - canvas_rect.top];
		if(document.getElementById("b_1").checked) { // add
			points_for_both.push(xy);
			both_i = points_for_both.length-1;
		} else { // move
			both_i = nearest_i(xy, points_for_both);
			points_for_both[both_i] = xy;
		}
		draw_canvas("both_canvas");
		canvas