paintBackground method 
    
      
  
void
paintBackground(
 - Context context
)override
     
    
    
  Implementation
  @override
void paintBackground(Context context) {
  if (data.isEmpty) {
    return;
  }
  final grid = Chart.of(context).grid;
  if (drawSurface) {
    _drawSurface(context, grid);
    if (surfaceOpacity != 1) {
      context.canvas
        ..saveContext()
        ..setGraphicState(
          PdfGraphicState(opacity: surfaceOpacity),
        );
    }
    context.canvas
      ..setFillColor(surfaceColor ?? color)
      ..fillPath();
    if (surfaceOpacity != 1) {
      context.canvas.restoreContext();
    }
  }
}