ntext.moveTo(20.5, sharp(y2s(0)));
		context.lineTo(-0.5, sharp(y2s(0)));
		context.moveTo(w - 20.5, sharp(y2s(0)));
		context.lineTo(w + 0.5, sharp(y2s(0)));
		context.moveTo(sharp(x2s(0)), h - 20.5);
		context.lineTo(sharp(x2s(0)), h + 0.5);
		context.moveTo(sharp(x2s(0)), 20.5);
		context.lineTo(sharp(x2s(0)), -0.5);
		context.stroke();
		context.closePath();
		context.setLineDash([]);

		// functions
		for(var i = 0; i < i_fs.length; ++i) {
			const f = i_fs[i];
			const color = i_colors[i];
			// draw
