Again, way too much code that has no bearing on the issue -- and you don't format your code so can be read on top of that -- we've been thru this discussion before, it seems...
All that is of significance here is
...
subplot(2,1,1)
plot(z, theta(1:round(nt/10):nt,:))
...
and if you don't want the first point, then don't ask for it ...
plot(z(2:end), theta(2:round(nt/10):nt,:))
or, if you really don't care about the initial points or know them from independent ICs, then just replace the first points with nan before calling plot...
subplot(2,1,1)
z(1)=nan;
theta(1,:)=nan;
plot(z, theta(1:round(nt/10):nt,:))
