How would I find the gradient of a best-fit line for input values on a scatterplot

Question

Based on this example, the below code plots a scatterplot to which double values can be input via a spinner. I'd like to plot a line of best fit on the scatter plot in order to calculate the gradient and find an average for the values which the value I get for gradient. I'd like it to be output onto a text box when a button is pressed. Help would be greatly appreciated.

package Grava;

        import javafx.application.Application;
        import javafx.geometry.Pos;
        import javafx.scene.Scene;
        import javafx.scene.control.Button;
        import javafx.scene.control.TextField;
        import javafx.scene.control.*;
        import javafx.scene.image.Image;
        import javafx.scene.layout.BorderPane;
        import javafx.scene.layout.HBox;
        import javafx.stage.Stage;
        import org.jfree.chart.ChartFactory;
        import org.jfree.chart.JFreeChart;
        import org.jfree.chart.fx.ChartViewer;
        import org.jfree.chart.plot.XYPlot;
        import org.jfree.chart.renderer.xy.XYLineAndShapeRenderer;
        import org.jfree.data.statistics.Regression;
        import org.jfree.data.xy.XYSeries;
        import org.jfree.data.xy.XYSeriesCollection;

public class ScatterAdd extends Application {


    private final XYSeries series = new XYSeries("Voltage");
    private final XYSeries trend = new XYSeries("Trend");
    private final XYSeriesCollection dataset = new XYSeriesCollection(series);

    ChoiceBox<String> domainLabels = new ChoiceBox<>();
    ChoiceBox<String> rangeLabels = new ChoiceBox<>();

    private JFreeChart createChart() throws Exception {
        /*
        JFreeChart chart = createChart();
        domainLabels.getSelectionModel().selectedItemProperty().addListener((ov, s0, s1) -> {
            chart.getXYPlot().getDomainAxis().setLabel(s1);
        });
        rangeLabels.getSelectionModel().selectedItemProperty().addListener((ov, s0, s1) -> {
            chart.getXYPlot().getRangeAxis().setLabel(s1);
        });
        XYPlot plot = createChart().getXYPlot();
        XYLineAndShapeRenderer r = (XYLineAndShapeRenderer) plot.getRenderer();
        r.setSeriesLinesVisible(1, Boolean.TRUE);
        r.setSeriesShapesVisible(1, Boolean.FALSE);
        this part gives me an error when i create it in the createChart() method
        */
         
        return ChartFactory.createScatterPlot("VI Characteristics", "Current", "Voltage", dataset);
        
    }

    @Override
    public void start(Stage stage) throws Exception {

        Image image = new Image("Grava.logo.png");
        stage.getIcons().add(image);

        var equation = new TextField();



        series.addChangeListener((event) -> {
            double[] coefficients = Regression.getOLSRegression(dataset, 0);
            double b = coefficients[0]; // intercept
            double m = coefficients[1]; // slope
            double x = series.getDataItem(0).getXValue();
            trend.add(x, m * x + b);
            x = series.getDataItem(series.getItemCount() - 1).getXValue();
            trend.add(x, m * x + b);
            dataset.addSeries(trend);
            equation.setText("y = " + m + " x + " + b);

        });




        domainLabels.getItems().addAll("Current", "Seconds");
        domainLabels.setValue("Current");

        rangeLabels.getItems().addAll("Voltage", "Metres");
        rangeLabels.setValue("Voltage");

        var xSpin = new Spinner<Double>(-10000000.000, 10000000.000, 0, 0.1);
        xSpin.setEditable(true);
        xSpin.setPromptText("Xvalue");

        var ySpin = new Spinner<Double>(-10000000.000, 10000000.000, 0, 0.1);
        ySpin.setEditable(true);
        ySpin.setPromptText("Yvalue");

        var button = new Button("Add");
        button.setOnAction(ae -> series.add(xSpin.getValue(), ySpin.getValue()));


        HBox xBox = new HBox();
        xBox.getChildren().addAll(domainLabels);

        HBox yBox = new HBox();
        yBox.getChildren().addAll(rangeLabels);

        var enter = new ToolBar(xBox, xSpin, yBox, ySpin, button, equation);
        BorderPane.setAlignment(enter, Pos.CENTER);

        BorderPane root = new BorderPane();
        root.setCenter(new ChartViewer(createChart()));
        root.setBottom(enter);

        stage.setTitle("ScatterAdd");
        stage.setScene(new Scene(root, 720, 480));
        stage.show();

    }

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

Answer 1

As shown here, call Regression.getOLSRegression() and display the result. You can do this in your Add button handler, as suggested in your question, or in a SeriesChangeListener, as shown below. This fragment simply prints the equation of the line, but you can update the display as desired.

private final XYSeries series = new XYSeries("Voltage");
private final XYSeries trend = new XYSeries("Trend");
private final XYSeriesCollection dataset = new XYSeriesCollection(series);
…
// https://stackoverflow.com/a/61398612/230513
series.addChangeListener((event) -> {
    double[] coefficients = Regression.getOLSRegression(dataset, 0);
    double b = coefficients[0]; // intercept
    double m = coefficients[1]; // slope
    System.out.println("y = " + m + " x + " + b);
});

Note that the series will need at least two values, and it should be added to the collection only once. In particular,

• Don't recreate the dataset in createChart()

• Don't recreate the choice boxes in start()

• Add a TextField to the toolbar and invoke setText() in the change listener; the displayed equation will update as points are added.

• Create an XYSeries named trend and add into the dataset, then update the series in the change listener.

    var equation = new TextField();
    series.addChangeListener((event) -> {
        …
        equation.setText("y = " + m + "x + " + b);
        double x = series.getDataItem(0).getXValue();
        trend.clear();
        trend.add(x, m * x + b);
        x = series.getDataItem(series.getItemCount() - 1).getXValue();
        trend.add(x, m * x + b);
    });