Spline interpolation with one endslope constrained, the other endslope free?

Hi, I'm using cubic spline interpolation in matlab, and find that either it free the slopes of two end points, or constrain the two endslopes to certain values, like:

yy = spline(x,[0 y 0], xx);

I'm wondering how to constrain the slope of just one end point, with the other free.

Any suggestions?

Thanks!