Lagrange Polynomial Interpolation. Example: http://jsfiddle.net/maccesch/jgU3Y/

lagrange.js

/**
 * 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
 * var l = new Lagrange(0, 0, 1, 1);
 * var 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));
 */
var Lagrange = function(x1, y1, x2, y2) {
	
	this.xs = [x1, x2];
	this.ys = [y1, y2];
	this.ws = [];
	this._updateWeights();
}

/**
 * Adds a new point to the polynomial. L(x) = y
 * @return {Number} The index of the added point. Used for changing the point. See changePoint.
 */
Lagrange.prototype.addPoint = function(x, y) {
	this.xs.push(x);
	this.ys.push(y);
	this._updateWeights();
	return this.xs.length-1;
}

/**
 * Changes a previously added point.
 */
Lagrange.prototype.changePoint = function(index, x, y) {
	this.xs[index] = x;
	this.ys[index] = y;
	this._updateWeights();
}

/**
 * Recalculate barycentric weights.
 */
Lagrange.prototype._updateWeights = function() {
	var k = this.xs.length;
	var w;
	
	for (var j = 0; j < k; ++j) {
		w = 1;
		for (var i = 0; i < k; ++i) {
			if (i != j) {
				w *= this.xs[j] - this.xs[i];
			}
		}
		this.ws[j] = 1/w;
	}
}

/**
 * Calculate L(x)
 */
Lagrange.prototype.valueOf = function(x) {
	var a = 0;
	var b = 0;
	var c = 0;
	
	for (var j = 0; j < this.xs.length; ++j) {
		if (x != this.xs[j]) {
			a = this.ws[j] / (x - this.xs[j]);
			b += a * this.ys[j];
			c += a;
		} else {
			return this.ys[j];
		}
	}
	
	return b / c;
}