How to make my graph longer?

a=(-10:2:10);
f=(5.*a-cos(2+a.^2)+sin(0.25.*a+2));
plot([a,f],'--'); 
hold on;
 osx=zeros(size(a));
 plot([a,f],'b--'); 
 plot(osx)
 hold off

So i have this code,for some reason I can't make my yellow( osx=zeros) be longer and it ends in the middle of the graph,while I need it to go further,if not endlessly

Answer 1

[Image Analyst]

Instead of osx, just use yline

yline(0, 'Color', 'y', 'LineWidth', 3);