memory leak while making graph of functions in calculator iPad - objective-c

Question

i am developing a scientific calculator for iPad . I am trying to draw functions creating a graphical representation . but i a have a memory leak when i draw a function of second degree for example x^2 . I was wondering if you could help solve the following memory leak,please? I would appreciate any help. The button that opens the screen with the function to be drawn has this code :
userIsInTheMiddleOfTypingANumber = YES;
graphVC.expression =[NSMutableArray arrayWithObject:[NSString
stringWithFormat:#"%#",#"$x**2"] ];
[self prepareToSolve];
graphVC=[[GraphViewController alloc] initWithNibName:#"GraphViewController"
bundle:Nil];
[self presentModalViewController:graphVC animated:YES];
Then in the GraphViewController i set the expression with the code:
- (void)viewDidLoad
{
[graphView reset];
[graphView setNeedsDisplay];
currentColumn = [NSMutableString string];
substitutions = [NSMutableDictionary dictionary];
eval = [DDMathEvaluator sharedMathEvaluator];
[currentColumn setString:[expression objectAtIndex:0]];
}
and then the method expressionResultForXValue leaks
- (double)expressionResultForXValue:(NSNumber*)x requestor:(GraphView *)graphView {
[substitutions setValue:x forKey:#"x"];
return [[eval evaluateString:currentColumn withSubstitutions:substitutions]
doubleValue]; //[CalculatorBrain evaluateExpression:self.expression
usingVariableValues:variableValues];
}
even though in the viewDidUnload method i set - [expression release];
The expressionResultForXValue method is called many times by the UIView GraphView by the drawrect method:
- (void)drawRect:(CGRect)rect {
CGPoint midpoint;
midpoint.x = self.bounds.origin.x + self.bounds.size.width/2 + originOffset.x;
midpoint.y = self.bounds.origin.y + self.bounds.size.height/2 + originOffset.y;
[[UIColor whiteColor] setStroke];
[AxesDrawer drawAxesInRect:self.bounds originAtPoint:midpoint
scale:self.scale*self.contentScaleFactor];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 5);
CGContextBeginPath(context);
for (CGFloat i = 0; i <= self.bounds.size.width; i++) {
CGPoint nextPointInViewCoordinates;
nextPointInViewCoordinates.x = i;
CGPoint nextPointInGraphCoordinates;
nextPointInGraphCoordinates.x = (nextPointInViewCoordinates.x - midpoint.x)/(self.scale
* self.contentScaleFactor);
nextPointInGraphCoordinates.y = ([self.delegate expressionResultForXValue:[NSNumber
numberWithDouble:nextPointInGraphCoordinates.x]
requestor:self]);
nextPointInViewCoordinates.y = midpoint.y - (nextPointInGraphCoordinates.y * self.scale
* self.contentScaleFactor);
if (i == 0) {
CGContextMoveToPoint(context, nextPointInViewCoordinates.x,
nextPointInViewCoordinates.y);
} else {
CGContextAddLineToPoint(context,
nextPointInViewCoordinates.x,nextPointInViewCoordinates.y);
}
}
[[UIColor yellowColor] setStroke];
CGContextDrawPath(context, kCGPathStroke);
}
The memory leak i get is :
Leaked Object # Address Size Responsible Library Responsible Frame
_DDFunctionExpression 1020 < multiple > 15,94 KB Rechemaschinn
+[DDExpression functionExpressionWithFunction:arguments:error:]
NSDecimalNumber 1018 < multiple > 15,91 KB Foundation -[NSDecimalNumberPlaceholder initWithDecimal:]
_DDNumberExpression 1020 < multiple > 15,94 KB Rechemaschinn +[DDExpression numberExpressionWithNumber:]
NSArray 1020 < multiple > 15,94 KB Rechemaschinn -[_DDFunctionExpression initWithFunction:arguments:error:]
_DDVariableExpression 1020 < multiple > 15,94 KB Rechemaschinn +[DDExpression variableExpressionWithVariable:]
__NSCFString 1019 < multiple > 15,92 KB Foundation -[NSPlaceholderString initWithCharacters:length:]
Thank you in advance.