gistfile1.java

    private DoubleBinding averageBinding = new FXDoubleBinding() {
        /**
         * This method is called every time the binding is becoming invalid.
         */
        @Override
        protected void configure() {
            addObservable(values());
            for (XYChart.Data<Number, Number> d : values()) {
                addObservable(d.XValueProperty());
            }
        }
        @Override
        protected double compute() {
            double result = 0;
            if (values().isEmpty()) {
                return result;
            }
            for (XYChart.Data<Number, Number> d : values()) {
                result += d.getXValue().doubleValue();
            }
            return result / values().size();
        }
    };
    //...
        public static void main(String[] args) {
        Foo foo = new Foo();
        System.out.println(foo.averageBinding().get()); //Should print 0.2
        foo.values().add(new XYChart.Data<Number, Number>(-3d, -3d));
        System.out.println(foo.averageBinding().get()); //Should print -0.6
        foo.values().get(0).setXValue(30d);
        System.out.println(foo.averageBinding().get()); //Should  print 6.825
        foo.values.get(3).setXValue(10d);
        System.out.println(foo.averageBinding().get()); //Should  print 10.075
    }