Interpolating polynomial for a set of points.
The polynomial passes through all the pairs (xi,yi). One may additionally specify a number of derivatives at each point xi; this is done by repeating the value xi and specifying the derivatives as successive yi values.
Allows evaluation of the polynomial and all its derivatives. For reasons of numerical stability, this function does not compute the coefficients of the polynomial, although they can be obtained by evaluating all the derivatives.
Parameters :  xi : arraylike, length N
yi : arraylike
axis : int, optional


Notes
Be aware that the algorithms implemented here are not necessarily the most numerically stable known. Moreover, even in a world of exact computation, unless the x coordinates are chosen very carefully  Chebyshev zeros (e.g. cos(i*pi/n)) are a good choice  polynomial interpolation itself is a very illconditioned process due to the Runge phenomenon. In general, even with wellchosen x values, degrees higher than about thirty cause problems with numerical instability in this code.
Based on [R21].
References
[R21]  (1, 2) Krogh, “Efficient Algorithms for Polynomial Interpolation and Numerical Differentiation”, 1970. 
Examples
To produce a polynomial that is zero at 0 and 1 and has derivative 2 at 0, call
>>> KroghInterpolator([0,0,1],[0,2,0])
This constructs the quadratic 2*X**22*X. The derivative condition is indicated by the repeated zero in the xi array; the corresponding yi values are 0, the function value, and 2, the derivative value.
For another example, given xi, yi, and a derivative ypi for each point, appropriate arrays can be constructed as:
>>> xi_k, yi_k = np.repeat(xi, 2), np.ravel(np.dstack((yi,ypi)))
>>> KroghInterpolator(xi_k, yi_k)
To produce a vectorvalued polynomial, supply a higherdimensional array for yi:
>>> KroghInterpolator([0,1],[[2,3],[4,5]])
This constructs a linear polynomial giving (2,3) at 0 and (4,5) at 1.
Methods
__call__(x)  Evaluate the interpolant 
derivative(x[, der])  Evaluate one derivative of the polynomial at the point x 
derivatives(x[, der])  Evaluate many derivatives of the polynomial at the point x 