Hi Bernd,
Greetings from Syncfusion. The EdgeLabelVisibility property is available only for Numerical, DateTime and logarithmic axis only. For Category and DateTimeCategory axis, EdgeLabelVisibility support is not provided. We will change our UG documentation accordingly and the changes will be refreshed in live after our 2019 Volume1 release which is scheduled to be rolled out in the mid of March.
Regards,
Bharathi.