public void drawCoordinateSystem(Graphics g) {
		g.setColor(Color.lightGray);
		g.drawLine(0, startY, PLOT_WIDTH, startY);
		g.drawLine(startX, 0, startX, PLOT_HEIGHT);

		for (int x = startX - (int) zoom; x > zoom; x -= (int) zoom) {
			String num = "" + (x - startX) / (int) zoom;
			g.drawString(num, x, startY);
		}

		for (int x = startX; x < PLOT_WIDTH - zoom; x += (int) zoom) {
			String num = "" + (x - startX) / (int) zoom;
			g.drawString(num, x, startY);
		}

		for (int y = startY - (int) zoom; y > zoom; y -= (int) zoom) {
			String num = "" + (startY - y) / (int) zoom;
			if (!num.equals("0"))
				g.drawString(num, startX, y);
		}

		for (int y = startY; y < PLOT_HEIGHT - zoom; y += (int) zoom) {
			String num = "" + (startY - y) / (int) zoom;
			if (!num.equals("0"))
				g.drawString(num, startX, y);
		}
	}