Description 
PP = PCHIPD(X,Y,D) provides the piecewise cubic polynomial which interpolates values Y and derivatives D at the sites X. This is meant to augment the builtin Matlab function PCHIP, which does not allow the user to specify derivatives.
X must be a vector.
If Y and D are vectors, then Y(i) and D(i) are the value and derivative to be matched at X(i).
If Y and D are matrices, then size(Y,2) == size(D,2) == length(X). Also, size(Y,1) == size(D,1). Use this for interpolating vector valued functions.
YY = PCHIPD(X,Y,D,XX) is the same as YY = PPVAL(PCHIPD(X,Y,D),XX), thus providing, in YY, the values of the interpolant at XX.
