The relative strength indicator will change the range of the axis, typically to a 0-100 range. If your spline series data is above or below that range, you will not see it.
Try using two separate Y axes, one for the spline series and one for the indicator.

Here's a quick example of having a spline and the indicator on the same chart, but different Y axes. Note, that the data isn't very useful, as there are only a few data points. The indicator needs to have quite a bit more points to make sense. This is primarily to illustrate the approach.

    NSArray *data = @[@5, @6, @3];
    NSArray *openValues = @[@5, @16, @15,];
    NSArray *closeValues = @[@15, @6, @23];
    NSArray *highValues = @[@20, @20, @29];
    NSArray *lowValues = @[@1, @0, @5];  

    _chart = [[IGChartView alloc]initWithFrame:self.view.frame];

    _categoryHelper = [[IGCategorySeriesDataSourceHelper alloc]initWithValues:data];

    _ohlcHelper = [[IGOHLCSeriesDataSourceHelper alloc]initWithOpenValues:openValues highValues:highValues lowValues:lowValues andCloseValues:closeValues];

    [_chart addSeriesForType:[IGSplineSeries class] usingKey:@"series" withDataSource:_categoryHelper firstAxisKey:@"x" secondAxisKey:@"y"];

    [_chart addSeriesForType:[IGRelativeStrengthIndexIndicator class] usingKey:@"rsi" withDataSource:_ohlcHelper firstAxisKey:@"x" secondAxisKey:@"y2"];

    [self.view addSubview:_chart];