Java

                                 Class: Main
package sample;

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.geometry.Insets;
import javafx.scene.chart.LineChart;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.XYChart;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Slider;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;
import java.io.OutputStream;
import java.io.IOException;

public class Main extends Application {
    private final static int MAX_MOISTURE_VALUE = 1<<10;

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

    @Override
    public void start(Stage stage) {
        var controller = new Controller(); //create the controller
        var serialPort = MinorProject.SerialPortService.getSerialPort("COM3");
        var outputStream = serialPort.getOutputStream();
        serialPort.addDataListener(controller);

        var pane = new BorderPane();

        var button = new Button("PUMP");

        var slider = new Slider();
        slider.setMin(0.0);
        slider.setMax(3000.0);

        button.setOnMousePressed(value -> {
            try {
                outputStream.write(255);
                System.out.println("Watering manually!");
            } catch (IOException e) {
                e.printStackTrace();
            }
        });
        button.setOnMouseReleased(value -> {
            try {
                outputStream.write(0);
            } catch (IOException e) {
                e.printStackTrace();
            }
        });

        slider.valueProperty().addListener((observableValue, oldValue, newValue) -> {
            try {
                outputStream.write(newValue.byteValue());
                if(newValue.byteValue()>0){
                    outputStream.write(110);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        });

        stage.setTitle("Automatic Watering System");

        var now = System.currentTimeMillis();

        var xAxis = new NumberAxis("Time Elapsed (seconds)", now, now + 50000, 10000); //creates the x-axis (which automatically updates)
        var yAxis = new NumberAxis("Voltage", 0, MAX_MOISTURE_VALUE, 10); //creates the y-axis

        var series = new XYChart.Series<>(controller.getDataPoints()); // creates the series (all the data)
        var lineChart = new LineChart<>(xAxis, yAxis, FXCollections.singletonObservableList(series)); //creates the chart
        lineChart.setTitle("Voltage from the Moisture Sensor");

        pane.setTop(button);
        pane.setBottom(lineChart);
        pane.setCenter(slider);
        pane.setPadding(new Insets(0, 20, 0, 20));

        Scene scene = new Scene(pane, 800,600); //creates the JavaFX window

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

    }


}

                                         class :Controller 
package sample;

import com.fazecast.jSerialComm.SerialPort;
import com.fazecast.jSerialComm.SerialPortEvent;
import com.fazecast.jSerialComm.SerialPortMessageListenerWithExceptions;
import javafx.application.Platform;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.scene.chart.XYChart;

import java.nio.ByteBuffer;

public class Controller implements SerialPortMessageListenerWithExceptions {

    private static final byte[] DELIMITER = new byte[]{'\n'};
    private final ObservableList<XYChart.Data<Number,Number>> dataPoints;

    public Controller() {
        this.dataPoints = FXCollections.observableArrayList();
    }

    public ObservableList<XYChart.Data<Number, Number>> getDataPoints() {
        return dataPoints;
    }

    @Override
    public int getListeningEvents() {
        return SerialPort.LISTENING_EVENT_DATA_RECEIVED;
    }

    @Override
    public void serialEvent(SerialPortEvent serialPortEvent) {
        if(serialPortEvent.getEventType() != SerialPort.LISTENING_EVENT_DATA_RECEIVED) {
            return;
        }
        byte[] data = serialPortEvent.getReceivedData();
        int rawB = ByteBuffer.wrap(data).getInt();
        long time = System.currentTimeMillis();
        var dataPoint = new XYChart.Data<Number,Number>(time, rawB);
        Platform.runLater(() -> this.dataPoints.add(dataPoint));
        // TODO: Implement this method. Refer to the documentation for more details.
    }

    @Override
    public void catchException(Exception e) {
        e.printStackTrace();
    }

    @Override
    public byte[] getMessageDelimiter() {
        return DELIMITER;
    }

    @Override
    public boolean delimiterIndicatesEndOfMessage() {
        return true;
    }

}

                                     Class : SerialPortService
package MinorProject;




import com.fazecast.jSerialComm.SerialPort;

public class SerialPortService {
    private SerialPortService() {}

    public static SerialPort getSerialPort(String portDescriptor) {
        var sp = SerialPort.getCommPort(portDescriptor);

        sp.setComPortParameters(9600, Byte.SIZE, SerialPort.ONE_STOP_BIT, SerialPort.NO_PARITY);
        sp.setComPortTimeouts(SerialPort.TIMEOUT_WRITE_BLOCKING, 0, 0);

        var hasOpened = sp.openPort();
        if (!hasOpened) {
            throw new IllegalStateException("Failed to open port.");
        }

        return sp;
    }
}