You may disable text wrapping for the X axis item labels:
RadChart1.PlotArea.XAxis.Appearance.TextAppearance.AutoTextWrap = Styles.AutoTextWrap.False;
You may also consider rotating the labels as shown in this help topic
Finally, you can control the text quality in RadChart like this:
RadChart1.Appearance.TextQuality = Telerik.Charting.Styles.TextQuality.AntiAlias;
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