It is easier to first create the lines and then make a single call to legend()
p = linspace(0,1,50);
v = -290*9.8*log(0.15+0.85*p);
v2 = -370*9.8*log(0.25+0.75*p);
plot(p,v,'-r');
hold on
p2 = plot(p,v2,'-b');
legend({'option1', 'option2'})
hold off
xlabel('PI for two options')
ylabel('staging velocity(km/s)')