X must be a vector.
The X vector should have real elements.
All data points with NaN as their site will be ignored.
There should be at least two data points.
The data sites should be distinct.
The endslopes cannot be NaN.
The endslopes cannot be Inf.
The number of sites, {0,number,integer}, is incompatible with the number of values, {1,number,integer}.
All data points with NaN in their value will be ignored.