CorePlot "indexOfVisiblePointClosestToPlotAreaPoint" giving me wrong index

Question

Currently I am plotting the graph with two data plots(two lines on the graph), when I click on one line "indexOfVisiblePointClosestToPlotAreaPoint" method giving me right index and other one throwing me wrong one, not even closest visible one its skipping/jumping multiple points in between. Ex: if I click on index number 20 it jumps to 15 or 25 vice versa. Here is the calculation to find the index

-(NSUInteger)indexOfVisiblePointClosestToPlotAreaPoint:(CGPoint)viewPoint
{
    NSUInteger dataCount = self.cachedDataCount;
    CGPoint *viewPoints  = calloc(dataCount, sizeof(CGPoint));
    BOOL *drawPointFlags = calloc(dataCount, sizeof(BOOL));

    [self calculatePointsToDraw:drawPointFlags forPlotSpace:(CPTXYPlotSpace *)self.plotSpace includeVisiblePointsOnly:YES numberOfPoints:dataCount];
    [self calculateViewPoints:viewPoints withDrawPointFlags:drawPointFlags numberOfPoints:dataCount];

    NSInteger result = [self extremeDrawnPointIndexForFlags:drawPointFlags numberOfPoints:dataCount extremeNumIsLowerBound:YES];
    if ( result != NSNotFound ) {
        CGFloat minimumDistanceSquared = CPTNAN;
        for ( NSUInteger i = (NSUInteger)result; i < dataCount; ++i ) {
            if ( drawPointFlags[i] ) {
                CGFloat distanceSquared = squareOfDistanceBetweenPoints(viewPoint, viewPoints[i]);
                if ( isnan(minimumDistanceSquared) || (distanceSquared < minimumDistanceSquared)) {
                    minimumDistanceSquared = distanceSquared;
                    result                 = (NSInteger)i;
                }
            }
        }
    }

    free(viewPoints);
    free(drawPointFlags);

    return (NSUInteger)result;
}