Center shadow and Mask

//Draw the shadow for center component
  CGContextSetFillColorWithColor( ctx, [UIColor colorWithWhite:.1 alpha:.4].CGColor );
  CGContextFillEllipseInRect( ctx, CGRectInset(self.bounds, CENTER_INSET-4, CENTER_INSET-4) );
  
  //Draw center transparent mask
  CGContextSetFillColorWithColor( ctx, [UIColor clearColor].CGColor );
  CGContextSetBlendMode(ctx, kCGBlendModeClear);
  CGContextFillEllipseInRect( ctx, CGRectInset(self.bounds, CENTER_INSET, CENTER_INSET) );
  

Circular Gradient

 // Get the graphics context and clear it
  CGContextRef ctx = UIGraphicsGetCurrentContext();
  CGContextClearRect(ctx, rect);
  
  // define stroke color
  CGContextSetRGBStrokeColor(ctx, 1, 1, 1, 1.0);
  
  // define line width
  CGContextSetLineWidth(ctx, 4.0);
  
  //Create fill gradient
  CGColorSpaceRef baseSpace = CGColorSpaceCreateDeviceRGB();
  CGGradientRef gradient = CGGradientCreateWithColorComponents(baseSpace, gradientColor, locations, 3);
  CGColorSpaceRelease(baseSpace), baseSpace = NULL;
  
  CGContextSaveGState(ctx);
  CGContextAddEllipseInRect(ctx, CGRectInset(rect, 0, 0));
  CGContextClip(ctx);
  
  CGPoint centerpoint = CGPointMake(rect.origin.x + (rect.size.width / 2), rect.origin.y + (rect.size.height / 2));
  CGContextDrawRadialGradient(ctx, gradient, centerpoint, 0, centerpoint, rect.size.height*.5, 0);
  CGGradientRelease(gradient), gradient = NULL;
  CGContextRestoreGState(ctx);

Pie Chart

// draw pie chart
  CGContextSetFillColorWithColor( ctx, _trackColor.CGColor );
  CGContextSetStrokeColorWithColor(ctx, _trackColor.CGColor);
  CGContextMoveToPoint(ctx, x, y);
  CGContextAddArc(ctx, x, y, x,  radians(start), radians(finish), 1);
  CGContextFillPath(ctx);
  CGContextStrokePath(ctx);