You can use rand() to produce a vector (or matrix/array) of random values between 0:1. I subtracted 0.5 from those random numbers to center them around 0. Then I scaled them by a factor of 2 to demonstrate how to scale the random noise. Then simply add the noise to your data.
xrand = (rand(size(x))-0.5) * 2;
yrand = (rand(size(Y))-0.5) * 2;
plot (x + xrand, Y + yrand, 'linewidth', 2)
By the way, you defined N twice in your code.
