Customized labels are not displayed [core-plot]
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I'm trying to draw a line chart using core-plot and having trouble with the x axis' labels .
Here is my code :

    graph = [[CPTXYGraph alloc] initWithFrame:self._graphView.bounds];
    
    CPTTheme *theme = [CPTTheme themeNamed:kCPTPlainWhiteTheme];
    
    [graph applyTheme:theme];
    
    CPTGraphHostingView *hostingView = (CPTGraphHostingView *)self._graphView;
    
    [hostingView setHostedGraph:graph];
    
    [self.view addSubview:hostingView];
    
    graph.paddingLeft = 10.0;
    graph.paddingTop = 10.0;
    graph.paddingRight = 10.0;
    graph.paddingBottom = 10.0;
    
    graph.plotAreaFrame.paddingLeft = 55.0;
    graph.plotAreaFrame.paddingTop = 50.0;
    graph.plotAreaFrame.paddingRight = 20.0;
    graph.plotAreaFrame.paddingBottom = 90.0;
    
    CPTXYPlotSpace *plotSpace = (CPTXYPlotSpace *)graph.defaultPlotSpace;
    
    plotSpace.xRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromInt(0) length:CPTDecimalFromInt([examValues count])];
    
    plotSpace.yRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromInt(0) length:CPTDecimalFromDouble([self getYRange] + 10)];
    
    CPTMutableLineStyle *lineStyle = [CPTMutableLineStyle lineStyle];
    lineStyle.lineColor = [CPTColor blackColor];
    lineStyle.lineWidth = 2.0f;
    
    CPTXYAxisSet *axisSet = (id)graph.axisSet;
    axisSet.xAxis.labelingPolicy = CPTAxisLabelingPolicyEqualDivisions;
    axisSet.xAxis.orthogonalCoordinateDecimal = CPTDecimalFromString(@"0");
    axisSet.xAxis.majorTickLineStyle = lineStyle;
    axisSet.xAxis.minorTickLineStyle = lineStyle;
    axisSet.xAxis.axisLineStyle = lineStyle;
    
    NSMutableArray *customLabels = [NSMutableArray arrayWithCapacity:[examValues count]];
    static CPTMutableTextStyle *labelTextStyle = nil;
    labelTextStyle = [[CPTMutableTextStyle alloc] init];
    labelTextStyle.color = [CPTColor blackColor];
    labelTextStyle.fontSize = 10.0f;
    
    int index = 1;
    for (SmartAssetInspectionResultHistoryDTO *result in examValues) {
        CPTAxisLabel *newLabel = [[CPTAxisLabel alloc] initWithText:result._inspectiondate textStyle:labelTextStyle];
        newLabel.tickLocation = CPTDecimalFromInt(index);
        newLabel.offset = axisSet.xAxis.labelOffset + axisSet.xAxis.majorTickLength;
        newLabel.rotation = M_PI / 4;
        [customLabels addObject:newLabel];
        [newLabel release];
        
        index ++;
    }
    
    axisSet.xAxis.axisLabels = [NSSet setWithArray:customLabels];
    
    axisSet.yAxis.majorIntervalLength = CPTDecimalFromString(@"20");
    axisSet.yAxis.orthogonalCoordinateDecimal = CPTDecimalFromInt(0);
    axisSet.yAxis.majorTickLineStyle = lineStyle;
    axisSet.yAxis.minorTickLineStyle = lineStyle;
    axisSet.yAxis.axisLineStyle = lineStyle;
    axisSet.yAxis.labelOffset = 3.0f;
    
    CPTScatterPlot *plot = [[[CPTScatterPlot alloc] init] autorelease];
    
    CPTMutableLineStyle *dataLineStyle = [CPTMutableLineStyle lineStyle];
    plot.identifier = @"Date Plot";
    
    dataLineStyle.lineWidth = 1.0f;
    dataLineStyle.lineColor = [CPTColor redColor];
    plot.dataLineStyle = dataLineStyle;
    plot.dataSource = self;
    
    CPTPlotSymbol *greenCirclePlotSymbol = [CPTPlotSymbol ellipsePlotSymbol];
    greenCirclePlotSymbol.fill = [CPTFill fillWithColor:[CPTColor greenColor]];
    greenCirclePlotSymbol.size = CGSizeMake(5.0, 5.0);
    plot.plotSymbol = greenCirclePlotSymbol;
    
    [graph addPlot:plot];
Some comments : 

    examValues <--- A NSMutableArray storing some data .
    SmartAssetInspectionResultHistoryDTO <--- Detail info object stored in examValues .
    result._inspectiondate <--- Date info I want to display in the x axis' labels .
My trouble : No dates are displayed below x axis but float values such like '1.0' , '2.0' ...
Who can tell me where the problem is ?