我设法用TimesPeriesCollection做了这件事

TimeSeries series = new TimeSeries("Cases per Day");
String url = "https://cnecovid.isciii.es/covid19/resources/datos_ccaas.csv";
String[] contents = HTTPFileDownloader.downloadFromURL(url).split("\n");

    for(String i : contents) {
        if(isCanaryIslandData(i)) {
            String[] line = i.split(",");
            String[] date = line[1].split("-");

            int year = Integer.parseInt(date[0]);
            int month = Integer.parseInt(date[1]);
            int day = Integer.parseInt(date[2]);

            int cases = Integer.parseInt(line[2]);

            series.add(new Day(day, month, year), cases);

        }
    }

然后在查特课上：

TimeSeriesCollection dataset = new TimeSeriesCollection();

    dataset.addSeries(series);

    JFreeChart chart = ChartFactory.createXYLineChart(
               "Line Chart",
               "x",
               "y",
               dataset,
               PlotOrientation.VERTICAL,
               true,
               true,
               false);

   ChartPanel chartPanel = new ChartPanel(chart);
   chartPanel.setMouseWheelEnabled(true);
   chartPanel.setPreferredSize(new java.awt.Dimension(560,367));

   XYPlot plot = (XYPlot) chart.getPlot();
   DateAxis dateAxis = new DateAxis();
   dateAxis.setDateFormatOverride(new SimpleDateFormat("dd-MM-yyyy"));
   plot.setDomainAxis(dateAxis);

和输出：

我不知道这是不是最好的解决办法，但确实有效。