Your derivative, Yprimenum, is by definition one element shorter than x, so you have to eliminate the last entry of x to plot it:
x= [0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 ]
y= [5 6 7 7.5 7.5 7.5 6.5 2.5 -5 -6 -6]
yprime=2.*x;
Yprimenum=zeros(1, length(x)-1);
for i= 1:length(x)-1;
Yprimenum(i)=(y(i+1)-y(i))./(x(i+1)-x(i));
end
figure;
hold on;
plot(x,y,'-b');
plot(x,yprime,'g');
plot(x(1:end-1),Yprimenum,'r');
hold off;
shg;
This is unavoidable with the method you used (and that the diff function uses) but there are ways to deal with it. This is one such.
