I think you need a little explanation on the scenario.
As I understood, you are using on both axes currencies, i.e. you are putting continuous numerical values. Thus, you do not need categories, since they are generated per item in the ItemsSource.
When using a categorical axis, you might have categories "Cat1" and "Cat3" in the ItemsSource, but this does not mean you have "Cat2" on the axis. But if you are using a linear axis, you will have all values between, let's say, 1 and 2 generated, i.e. 1.002, 1.0021 and so on. Consequently, you will be able to put an annotation on a specific value of your preference. Otherwise, you will need an already existing category, which, for instance, in your scenario is missing.
I hope this brings light into the behavior of RadChartView.
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