AFreeChart －最低限のグラフを描く

１．グラフのViewを準備するクラス

public class GraphView extends View {

    private AFreeChart chart;
    private RectShape chartArea;

    public GraphView(Context context, AttributeSet attrs) {
        super(context, attrs);
        chartArea = new RectShape();
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        chartArea.setWidth(w);
        // chartArea.setHeight(h / 2);
        chartArea.setHeight(h);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        this.chart.draw(canvas, chartArea);
    }

    public void setChart(AFreeChart chart) {
        this.chart = chart;
    }

}

２．時系列グラフを描くクラス

public class SampleChart {

    public void createChart(View view) {

        TimeSeries series = new TimeSeries("時系列データ");
        series.add(new Month(7, 2007), 120);
        series.add(new Month(8, 2007), 140);
        series.add(new Month(9, 2007), 150);
        series.add(new Month(10, 2007), 190);
        series.add(new Month(11, 2007), 230);
        series.add(new Month(12, 2007), 280);

        TimeSeriesCollection data = new TimeSeriesCollection(series);

        AFreeChart chart =
                ChartFactory.createTimeSeriesChart("PV推移",
                        "月",
                        "PV",
                        data,
                        false, // ← 凡例無し
                        false,
                        false);

        GraphView graphView = (GraphView) view;
        graphView.setChart(chart);

    }

}

３．レイアウト.xml

<com.jimdo.uchida001tmhr.graph.GraphView
    android:id="@+id/sample_chart"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"/>

 ４．MainActivity

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        View view = findViewById(R.id.sample_chart);
        SampleChart sampleChart = new SampleChart();
        sampleChart.createChart(view);

    }
                     (中略)
}

ここで、findViewById()をcreateChart()の中に入れてしまうと、NullPointerExceptionになってしまう。