y2s(y_i)));
				context.lineTo(w, sharp(y2s(y_i)));
			}
		}
		context.strokeStyle = "#AAA";
		context.lineWidth = 1;
		context.stroke();
		context.setLineDash([]);
		context.closePath();

		// verticals
		context.strokeStyle = "#333";
		context.lineWidth = 1;
		context.setLineDash([4, 4]);
		for(var i = 0; i < i_verticals.length; ++i) {
			context.beginPath();
			context.moveTo(sharp(x2s(i_verticals[i])), 0);
			context.lineTo(sharp(x2s(i_verticals[i])), h+1);
			context.stroke();
			context.closePath();
	