// The unstacked yval is equal to the current yval minus the yval of the
// next point at the same xval.
for (var i = idx+1; i < this.points.length; i++) {
// The unstacked yval is equal to the current yval minus the yval of the
// next point at the same xval.
for (var i = idx+1; i < this.points.length; i++) {