code: function Lagrange(x1, y1, x2, y2) {
    this.xs = [x1, x2];
    this.ys = [y1, y2];
    this.ws = [];
    this.updateWeights();
  }
docs: At least two points are needed to interpolate something.
  @class Lagrange polynomial interpolation.
  The computed interpolation polynomial will be reffered to as L(x).
  @example
  let l = new Lagrange(0, 0, 1, 1);
  let index = l.addPoint(0.5, 0.8);
  console.log(l.valueOf(0.1));
  
  l.changePoint(index, 0.5, 0.1);
  console.log(l.valueOf(0.1));
  
  @see https://gist.github.com/dburner/8550030
  @see http://jsfiddle.net/maccesch/jgU3Y/