x = linspace(0,pi);
y1 = cos(x);
plot(x,y1)
hold on
y2 = cos(2*x);
plot(x,y2)
y3 = cos(3*x);
plot(x,y3)
y4 = cos(4*x);
plot(x,y4)
hold off
legend({'cos(x)','cos(2x)','cos(3x)','cos(4x)'},'Location','northwest','NumColumns',2,'FontSize',12,)
如果不想显示某一个图例，可以参考以下代码：
figure(1);hold on;hi=plot([val_g,val_g],[0,g(val_g)],'r-','linewidth',1); set(hi(1),'handlevisibility','off');