Indeed this is not explained in the best way in the documentation and I will make sure it is updated.
On your question - the Y axis (if not configured explicitly) autoadjusts to accommodate the values the series have. These are all numerical values and they depend on the series' values, which is why they cannot be taken explicitly from the datasource. What you can do is to set the MinValue and MaxValue from the code-behind according to your data. The DataFormatString for the labels on the Y axis series is also located in the LabelsAppearance inner tag and you can see it in action in this online demo
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