paint method
Draw itself and its children, according to the calculated
box.offset
Implementation
@override
void paint(Context context) {
  super.paint(context);
  if (data.isEmpty) {
    return;
  }
  final grid = Chart.of(context).grid;
  if (drawLine) {
    _drawLine(context, grid, true);
    context.canvas
      ..setStrokeColor(lineColor ?? color)
      ..setLineWidth(lineWidth)
      ..setLineCap(PdfLineCap.round)
      ..setLineJoin(PdfLineJoin.round)
      ..strokePath();
  }
}