The axis is drawn according to the graph in a curve

I have a graph and I want to add two blue axes to this graph, in addition to the x-axis and the y-axis. However, these blue axes appear below the plot. Any idea how to get the blue axes over the top of the plot? Thank you in advance.

    blueAxisList = [[NSMutableDictionary alloc] init];


    CPTXYAxis *blueAxis = [blueAxisList objectForKey:name];
    blueAxis = [[CPTXYAxis alloc] init];
    blueAxis.coordinate = CPTCoordinateX;

    blueAxis.plotSpace = plotSpace;

    CPTMutableLineStyle *lineStyle = [CPTMutableLineStyle lineStyle];

    lineStyle.lineWidth = 3.f;

    blueAxis.axisLineStyle = lineStyle;
    blueAxis.minorTickLineStyle = nil;
    blueAxis.majorTickLineStyle = nil;
    blueAxis.labelingPolicy = CPTAxisLabelingPolicyNone;

    // Line cap on lhs
    CPTLineCap *lineCap = [CPTLineCap lineCap];
    lineCap.size      = CGSizeMake(8., 60.);
    lineCap.lineStyle = lineStyle;

    lineCap.lineCapType = CPTLineCapTypeRectangle;
    const double axisLength = plotSpace.xRange.lengthDouble;
    blueAxis.visibleAxisRange = [CPTPlotRange plotRangeWithLocation:plotSpace.xRange.location length:CPTDecimalFromDouble(axisLength)];
    blueAxis.axisLineCapMax = lineCap;

    NSMutableArray *axesList= [graph.axisSet.axes mutableCopy];

    [axesList addObject: blueAxis];
    graph.axisSet.axes = axesList;