code: function angleFromThreePoints (ax, ay, bx, by, cx, cy) {
    var c = distance(ax, ay, bx, by)
    var a = distance(bx, by, cx, cy)
    var b = distance(ax, ay, cx, cy)
    return Math.acos((a * a + c * c - b * b) / (2 * a * c))
  }
docs: /*
  Compute angle formed by three points in cartesian plane using law of cosines