How to add a value marker to JavaFX chart?

To convert chart values to pixels you can use method NumberAxis#getDisplayPosition() which return actual coordinates of the chart nodes.

Although these coordinates are relative to chart area, which you can find out by next code:

Node chartArea = chart.lookup(".chart-plot-background");
Bounds chartAreaBounds = chartArea.localToScene(chartArea.getBoundsInLocal());

Note localToScene() method which allows you to convert any coordinates to Scene ones. Thus you can use them to update your value marker coordinates. Make sure you make localToScene call after your Scene have been shown.

See sample program below which produces next chart:

public class LineChartValueMarker extends Application {

    private Line valueMarker = new Line();
    private XYChart.Series<Number, Number> series = new XYChart.Series<>();
    private NumberAxis yAxis;
    private double yShift;

    private void updateMarker() {
        // find maximal y value
        double max = 0;
        for (Data<Number, Number> value : series.getData()) {
            double y = value.getYValue().doubleValue();
            if (y > max) {
                max = y;
            }
        }
        // find pixel position of that value
        double displayPosition = yAxis.getDisplayPosition(max);
        // update marker
        valueMarker.setStartY(yShift + displayPosition);
        valueMarker.setEndY(yShift + displayPosition);
    }

    @Override
    public void start(Stage stage) {
        LineChart<Number, Number> chart = new LineChart<>(new NumberAxis(0, 100, 10), yAxis = new NumberAxis(0, 100, 10));

        series.getData().add(new XYChart.Data(0, 0));
        series.getData().add(new XYChart.Data(10, 20));

        chart.getData().addAll(series);
        Pane pane = new Pane();
        pane.getChildren().addAll(chart, valueMarker);
        Scene scene = new Scene(pane);

        // add new value on mouseclick for testing
        chart.setOnMouseClicked(new EventHandler<MouseEvent>() {
            @Override
            public void handle(MouseEvent t) {
                series.getData().add(new XYChart.Data(series.getData().size() * 10, 30 + 50 * new Random().nextDouble()));
                updateMarker();
            }
        });

        stage.setScene(scene);
        stage.show();

        // find chart area Node
        Node chartArea = chart.lookup(".chart-plot-background");
        Bounds chartAreaBounds = chartArea.localToScene(chartArea.getBoundsInLocal());
        // remember scene position of chart area
        yShift = chartAreaBounds.getMinY();
        // set x parameters of the valueMarker to chart area bounds
        valueMarker.setStartX(chartAreaBounds.getMinX());
        valueMarker.setEndX(chartAreaBounds.getMaxX());
        updateMarker();
    }

    public static void main(String[] args) {
        launch();
    }
}