spline takes data in the format ( known X's, known Y's, desired X's)
So, for example:
desiredx=1:0.1:13;
xfitted = spline(1:numel(x), x, desiredx);
plot(desired, xfitted);
That's just a simple cubic spline. See also 'pchip', which has the same API. Smoothing splines more formally have a 'pressure' to penalise excessive curvature. If you need them, they're in the Curve Fitting Toolbox.
