JavaFX sample to display a LineChart which shows the value of a plotted Node when you hover over the Node.

LineChartWithHover.java

import javafx.application.Application;
import javafx.collections.*;
import javafx.event.EventHandler;
import javafx.scene.*;
import javafx.scene.chart.*;
import javafx.scene.control.Label;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

/** Displays a LineChart which displays the value of a plotted Node when you hover over the Node. */
public class LineChartWithHover extends Application {
  @SuppressWarnings("unchecked")
  @Override public void start(Stage stage) {
    final LineChart lineChart = new LineChart(
        new NumberAxis(), new NumberAxis(),
        FXCollections.observableArrayList(
            new XYChart.Series(
                "My portfolio",
                FXCollections.observableArrayList(
                    plot(23, 14, 15, 24, 34, 36, 22, 45, 43, 17, 29, 25)
                )
            )
        )
    );
    lineChart.setCursor(Cursor.CROSSHAIR);

    lineChart.setTitle("Stock Monitoring, 2013");

    stage.setScene(new Scene(lineChart, 500, 400));
    stage.show();
  }

  /** @return plotted y values for monotonically increasing integer x values, starting from x=1 */
  public ObservableList<XYChart.Data<Integer, Integer>> plot(int... y) {
    final ObservableList<XYChart.Data<Integer, Integer>> dataset = FXCollections.observableArrayList();
    int i = 0;
    while (i < y.length) {
      final XYChart.Data<Integer, Integer> data = new XYChart.Data<>(i + 1, y[i]);
      data.setNode(
          new HoveredThresholdNode(
              (i == 0) ? 0 : y[i-1],
              y[i]
          )
      );

      dataset.add(data);
      i++;
    }

    return dataset;
  }

  /** a node which displays a value on hover, but is otherwise empty */
  class HoveredThresholdNode extends StackPane {
    HoveredThresholdNode(int priorValue, int value) {
      setPrefSize(15, 15);

      final Label label = createDataThresholdLabel(priorValue, value);

      setOnMouseEntered(new EventHandler<MouseEvent>() {
        @Override public void handle(MouseEvent mouseEvent) {
          getChildren().setAll(label);
          setCursor(Cursor.NONE);
          toFront();
        }
      });
      setOnMouseExited(new EventHandler<MouseEvent>() {
        @Override public void handle(MouseEvent mouseEvent) {
          getChildren().clear();
          setCursor(Cursor.CROSSHAIR);
        }
      });
    }

    private Label createDataThresholdLabel(int priorValue, int value) {
      final Label label = new Label(value + "");
      label.getStyleClass().addAll("default-color0", "chart-line-symbol", "chart-series-line");
      label.setStyle("-fx-font-size: 20; -fx-font-weight: bold;");

      if (priorValue == 0) {
        label.setTextFill(Color.DARKGRAY);
      } else if (value > priorValue) {
        label.setTextFill(Color.FORESTGREEN);
      } else {
        label.setTextFill(Color.FIREBRICK);
      }

      label.setMinSize(Label.USE_PREF_SIZE, Label.USE_PREF_SIZE);
      return label;
    }
  }

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

Answer to the StackOverflow question: LineChart Hover Values

Thanks~ Checking whether this solution is still instructive. By the way, checking for https://github.com/fair-acc/chart-fx instead of https://github.com/openjdk/jfx actually.