gistfile1.java

    /**
     * the average value of all XValue of Data in values
     *
     * @see #values()
     */
    private DoubleBinding averageBinding = new DoubleBinding() {
        {
            //call #computeValue every time #values() changes
            super.bind(values());
            for (XYChart.Data<Number, Number> d : values()) {
                super.bind(d.XValueProperty());
            }

            //bind (or unbind) new XValueProperty
            values().addListener(new ListChangeListener<XYChart.Data<Number, Number>>() {
                @Override
                public void onChanged(Change<? extends XYChart.Data<Number, Number>> change) {
                    while (change.next()) {
                        for (XYChart.Data<Number, Number> d : change.getAddedSubList()) {
                            bind(d.XValueProperty());
                        }
                        for (XYChart.Data<Number, Number> d : change.getRemoved()) {
                            unbind(d.XValueProperty());
                        }
                    }
                }
            });
        }
        @Override
        protected double computeValue() {
            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
    }