The barycentric interpolation algorithm allows easy updating by
adding more points for the polynomial to pass through.
xi : array_like
The x coordinates of the points that the polynomial should pass
yi : array_like, optional
The y coordinates of the points the polynomial should pass through.
Should have shape (xi.size,R); if R > 1 then the polynomial is
If yi is not given, the y values will be supplied later. yi should
be given if and only if the interpolator has y values specified.