Look at the attached movie. I change font to bigger one. And after that I point on the axis. You can see, how neighbor labels started to intersect. I change upperbound a bit, and reevaluating happens. Some labels disappear. after that I change font back and font actually become smaller. some space for missed labels appear, but labels didn't appear. I have to change upperbound (actually, I need somehow make axis to redraw itself), and missed labels appeared again.

Code:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.chart.LineChart;
import javafx.scene.chart.NumberAxis;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Slider;
import javafx.scene.layout.Pane;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;

public class JavaCharts extends Application {

    public static void main(String[] args) {
        launch(args);
    }
    NumberAxis axis1 = new NumberAxis(0, 100, 10);
    NumberAxis axis2 = new NumberAxis(0, 100, 10);
    LineChart testedLineChart = new LineChart(axis1, axis2);
    VBox vb = new VBox();

    @Override
    public void start(Stage stage) throws Exception {
        testedLineChart.setTitle("LineChart");
        testedLineChart.setStyle("-fx-border-color: darkgray;");

        Pane pane = new Pane();
        pane.setPrefSize(600, 600);

        Slider slider = new Slider(0, 500, 10);
        slider.setShowTickLabels(true);
        slider.setShowTickMarks(true);
        slider.valueProperty().bindBidirectional(axis1.upperBoundProperty());

        ComboBox cb = new ComboBox();
        cb.getItems().addAll(new Font("Arial", 20), Font.getDefault());
        cb.valueProperty().bindBidirectional(axis1.tickLabelFontProperty());

        pane.setPrefSize(600, 600);
        pane.getChildren().add(testedLineChart);

        vb.getChildren().addAll(pane, slider, cb);

        Scene scene = new Scene(vb, 700, 700);
        stage.setScene(scene);
        stage.show();
    }
}