This view draws a partially colored image as a pie chart based on two numeric values representing the total allowance and the current amount as a percentage of the total, using color and grayscale versions of an image.

ChartIconView

/*
 *
 *	This view draws a partially colored image as a pie chart based on two numeric values
 *	representing the total allowance and the current amount as a percentage of the total,
 *	using color and grayscale versions of an image.
 *
 */

#define PI 3.14159265358979323846

@interface ChartIconView: UIView 
@property (nonatomic, strong) UIImage *colorImage;
@property (nonatomic, strong) UIImage *grayImage;
@property (nonatomic, assign) float currentAmount;
@property (nonatomic, assign) float totalAllowance;
@end

@implementation ChartIconView

@synthesize colorImage = _colorImage;
@synthesize grayImage = _grayImage;
@synthesize currentAmount = _currentAmount;
@synthesize totalAllowance = _totalAllowance;

static inline float radians(double degrees) { return degrees * PI / 180; }
 
- (void)drawRect:(CGRect)rect {
    // Drawing code
	CGRect parentViewBounds = self.bounds;
	CGFloat x = CGRectGetWidth(parentViewBounds)/2;
	CGFloat y = CGRectGetHeight(parentViewBounds)/2;
	int radius = floor(x)+5;
	float endpos = (_currentAmount < _totalAllowance) ? ((_currentAmount / _totalAllowance) * 360.0) : 360.0;
 
	// Get the graphics context
 	CGContextRef ctx = UIGraphicsGetCurrentContext();
	CGContextSaveGState(ctx);
 
	CGContextSetRGBFillColor(ctx, 1.0, 1.0, 1.0, 1.0);
	CGContextFillRect(ctx, parentViewBounds);
    
	CGContextTranslateCTM(ctx, 0, parentViewBounds.size.height);
	CGContextScaleCTM(ctx, 1.0, -1.0);
    
	CGContextDrawImage(ctx, parentViewBounds, _grayImage.CGImage);
 
	CGContextMoveToPoint(ctx, x+2, y);     
	CGContextAddArc(ctx, x+2, y, radius, 0.0, radians(endpos), 0); 
	CGContextClosePath(ctx); 
 
	CGContextClip(ctx);
    
	CGContextDrawImage(ctx, parentViewBounds, _colorImage.CGImage);
 
	CGContextRestoreGState(ctx);
}

@end