You don't need a loop, you can use Matlab's colon operator to define a range of values first_value:step_size:last_value. You can also use linspace(first_value, last_value, Npoints).
y = rand(1, 500000); % fake some data
Npoints = numel(y);
delta = 0.005;
x_max = delta*Npoints - delta;
x = 0:delta:x_max;
or
x = linspace(0, x_max, Npoints);
