RadChart does not support such feature. Sorry for the caused inconvenience.
Nevertheless, I can offer you the following options as a workaround to improve the situation.
The first one is to set the LabelStep property. This way you will have less labels on the axis and they will not overlap.
radChart.PlotArea.XAxis.LabelStep = 2;
The other one is to have the label on more than one line. This is achieved by setting the CustomFormat property.
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