public class RightMenu extends GridPane {

public RightMenu() {

    Button startButton = new Button("Старт");
    Button stopButton = new Button("Стоп");
    Button clearButton = new Button("Очистить график");

    this.add(startButton, 0, 0);
    this.add(stopButton,  1, 0);
    this.add(clearButton,  0, 1);

    this.setPadding(new Insets(2, 2, 2, 2));
}

GridPane.setColumnSpan(clearButton,2);

GridPane.setHalignment(clearButton, HPos.CENTER);

VBox vbox = new VBox();
HBox hbox = new HBox();

hbox.getChildren().addAll(startButton, stopButton);

vbox.getChildren().addAll(hbox, clearButton);