x1 = linspace(6, 8.81);
y1 = 0.3*(10^6)*(3^0.5)*((x1.^2)./36-1);%这条线当成图例的Y1
y2 = 0.3*(10^6)*(3^0.5)*((x1.^2)./12-1);%这条线当成图例的Y2
y3 = 3*8.81^2./(1000*2*x1)*(17.2-3*3^0.5*(8.81^2/36-1));%这条线当成图例的Y3
x2 = linspace(8.81, 55);
y11 = 1.72*(10^6)*(1-8.81^2./(x2.^2))+(3*3^0.5*10^5)*(8.81^2./(6^2)-1)*8.81^2./(x2.^2);
y22 = 1.72*(10^6)*(1+8.81^2./(x2.^2))-(3*3^0.5*10^5)*(8.81^2./(6^2)-1)*8.81^2./(x2.^2);
y33 = 3*8.81^2./(1000*2*x2)*(17.2-3*3^0.5*(8.81^2/36-1));
yyaxis left; % 激活左边的轴
plot(x1,y1,'b',x1,y2,'g',x2,y11,'b',x2,y22,'g');
title('shress diagram');
yyaxis right; % 激活右边的轴
plot(x1,y3,'r',x2,y33,'r');
%ylim([-150,150]); % 设置右y轴的界限
hold on;
plot([8.81 8.81], get(gca, 'YLim'), '--m', 'LineWidth', 1) % 紫色,宽度为3
legend('y1','y2','y3')%图例的Y3本应该是红色,但是怎么写都是黑色T.T