drawInitailPoint method
desenha o ponto inicial de cruzamento os dois eixos
Implementation
void drawInitailPoint(Canvas canvas, Size size) {
final paint = Paint()
..color = const Color.fromARGB(255, 0, 0, 0)
..strokeWidth = 3
..style = PaintingStyle.fill;
canvas.drawCircle(Offset(30, size.width - 30), 2, paint);
}