Why can't I add plots to axes after creating a legend?

I was following the instructions here https://uk.mathworks.com/help/matlab/ref/legend.html. However, when I run the following code (is the same as one provided in the link) -

x = linspace(0,10);
y1 = sin(x);
plot(x,y1)
legend('First Line')
hold on
y2 = cos(x);
plot(x,y2,'DisplayName','Second Line')

I get the attached graph. Could someone please tell me why this is happening? Thank you!

Answer 1

[Image Analyst]

legend() does not automatically update when you add stuff. How could it know what you want to call the additional curves? I don't know what the 'DisplayName' option is in plot(). My R2017a help for plot does not mention that as a possible option. Is that what you thought legend() would automatically use? Evidently not. You'll have to call legend after you're all done plotting curves, not before then.