Applying diff to a Nx1 array produces an (N-1)x1 array, hence your problem.
In truth, you are not really calculating the derivative of the array, because the function diff only calculates the difference of each successive pair of elements. You must divide that by 
if you wish to compute the derivative.
if you wish to compute the derivative.Still, the derivative array will have one less element than the original vector, so you can plot it only vs x(1:end-1)(depending on how you define the differencing scheme).
See the example below for reference
x=1:10;
y1=sin(2*x);
y2=diff(y1)./diff(x);
plot(x,y1,x(1:end-1),y2)
legend('function','derivative')
Now, if the derivative must have the same number of elements of the original array, some more sophisticated techniques can be applied, but that would be a whole different subject.
