drawGrid method 
    
      
  
void
drawGrid(
 - Canvas canvas, 
- int gridRows, 
- int gridColumns
)override
     
    
    
  Implementation
  @override
void drawGrid(Canvas canvas, int gridRows, int gridColumns) {
  canvas.drawLine(Offset(0, chartRect.top), Offset(chartRect.width, chartRect.top), gridPaint);
  canvas.drawLine(Offset(0, chartRect.bottom), Offset(chartRect.width, chartRect.bottom), gridPaint);
  double columnSpace = chartRect.width / gridColumns;
  for (int i = 0; i <= columnSpace; i++) {
    //mSecondaryRect垂直线
    canvas.drawLine(Offset(columnSpace * i, chartRect.top - style.padding.top), Offset(columnSpace * i, chartRect.bottom), gridPaint);
  }
}