I see now what you mean. Your chart setup uses the automatic AxisY range and when all data is zero, the AxisY does not have any range (i.e. it is zero).
We would suggest you to manually set the AxisY range like this to have YAxis Labels visible. Feel free to customize the values of the range as you wish.
radChart1.DefaultView.ChartArea.AxisY.AutoRange = false;
radChart1.DefaultView.ChartArea.AxisY.AddRange(0, double.Epsilon, double.Epsilon);
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