Each element of v is calculated from two elements from y and two elements from t. Which is to say, each element of v is calculated from an initial time and displacement and a final time and displacement.
So, as you noted in your comment, you get one fewer v value than you had t and y values. That's a consequence of each v requiring 2 t's and 2 y's.
The question then is: Which t should each v be associated with (for plotting or whatever)? Should each v be associated with its initial time or its final time or maybe the time halfway between its initial and final time?
Since here t is a discrete variable (not continuous like we tend to think of it), it is pretty much arbitrary how you decide to associate each v with a t: each velocity is defined over a time interval and each velocity is associated with that whole interval, so how you depict it is up to you.
Here are a few plots, for visual aid (and you can pick the one that makes the most sense and use that code):
title('v vs t_{initial}')
plot((t(1:end-1)+t(2:end))/2,v,'.-')
plot(t([1 repelem(2:end-1,1,2) end]),repelem(v,1,2))
title('v vs t_{interval}')