Lagrange(x,y,x_inte​r)

This function is implemented for calculating Lagrange interpolation polynomial

Lagrange interpolation is a kind of polynomial interpolations.
The nth-order polynomial interpolation requires n+1 points.
This method yields the same result as Newton's divided differences interpolation polynomial.
However, in my point of view, it is easier to write a code.
Reference: https://en.wikipedia.org/wiki/Lagrange_polynomial