When a large number of data points are used to draw a straight line in the chart, the rendered line is not straight.
For example:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.chart.LineChart;
import javafx.scene.chart.NumberAxis;
import javafx.scene.chart.XYChart;
import javafx.stage.Stage;

public class ChartBug extends Application {
    
    @Override
    public void start(Stage primaryStage) {
        //defining the axes
        final NumberAxis xAxis = new NumberAxis();
        final NumberAxis yAxis = new NumberAxis();

        //creating the chart
        final LineChart<Number,Number> lineChart =
                new LineChart<>(xAxis,yAxis);
        // Disable symbols
        lineChart.setCreateSymbols(false);
        //defining a series
        XYChart.Series series = new XYChart.Series();
        for(int i=0; i<=100; i++)
        {
            series.getData().add(new XYChart.Data<>(i,i));
        }
        lineChart.getData().add(series);
        
        Scene scene = new Scene(lineChart,800,600);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }
    
}

This produces the following chart : http://imgur.com/ddSQT9i,lQ8nNMx#0

While changing
for(int i=0; i<=100; i++)
        {
            series.getData().add(new XYChart.Data<>(i,i));
        }

To
for(int i=0; i<=10; i++)
        {
            series.getData().add(new XYChart.Data<>(10*i,10*i));
        }

produces a straight line as expected. http://imgur.com/ddSQT9i,lQ8nNMx#1