You need to plot sx and sy separately, and don't redefine t after you've used it in your equations.
Not sure why you had the time as that, but I guess it's some equation you got in class or from a book.
v0 = input('Enter Initial Velocity of Ball in m/s and Press Return: ');
vx = v0.*cos(angle*(pi/180));
vy = v0.*sin(angle*(pi/180));
sx = vx.*t;
sy = vy.*t-0.5*g.*t.^2;
plot(t, sx, 'LineWidth', 2);
plot(t, sy, 'LineWidth', 2);
xlabel('time', 'FontSize', 20)
ylabel('sy or sy', 'FontSize', 20)
See my attached demo, that I've posted many times, and you can find by searching the tags projectile and trajectory. It computes just about everything you could want and allows you to set just about every initial condition you could ever want.