paint method
Draw itself and its children, according to the calculated
box.offset
Implementation
@override
void paint(Context context) {
  super.paint(context);
  final datasets = Chart.of(context).datasets;
  clip(context, box!.size);
  for (final dataSet in datasets) {
    dataSet.paintBackground(context);
  }
  context.canvas.restoreContext();
  paintBackground(context);
  clip(context, box!.size);
  for (final dataSet in datasets) {
    dataSet.paint(context);
  }
  context.canvas.restoreContext();
  _xAxis.paint(context);
  _yAxis.paint(context);
  for (final dataSet in datasets) {
    dataSet.paintForeground(context);
  }
}