Spline with non-increasing x values

Hi, currently I have a problem with creating cubic spline that has non-increasing x values. I created 2 arrays X[i] and Y[i], and I pass them through a spline routine created by John Burkardt, it is able to work but I need to be able to create spline that intersects or can go in the opp direction. I thought about splitting the process etc, i scan through my X array and when I come to a point where there is a change in direction of X, i stop and pass that whole part through the spline routine, then i continue and when i come to another change in direction, i pass that part through the routine again... but I feel that not only that it will be tedious and slow but the end results, the curve will not be smooth. Anyone have any solutions? I have appended the code of the spline routine and evaluation.
Thanks for the help

Mark