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;
}

CodePudding user response：

Are you checking the plot parameter passed in the delegate method? The touch will register on the frontmost plot if there are any points within the plotSymbolMarginForHitDetection. It won't even check the other plot unless nothing hits on the front one. With two lines close together like that, you'll need to use a small hit margin to make sure touches register on the right plot.