帮我写一段matlab代码，将三个关于x的函数式画在一个图里，函数关系式分别是y2=1-exp[-(x/128)^5.21]，y3=1-0.83(1.2-0.2x^5)，y4=1-0.83/（0.83+0.17x^5.73）

x = 0:0.01:256;
y2 = 1-exp(-(x/128).^5.21);
y3 = 1-0.83*(1.2-0.2*x.^5);
y4 = 1-0.83./(0.83+0.17*x.^5.73);
plot(x, y2, x, y3, x, y4);
legend('y2', 'y3', 'y4');
xlabel('x');
ylabel('y');
title('Three Functions of x');