To be able to set custom format for Axis Labels of RadChart you should use ValueFormat property as it automatically formats axis label values as Currency, Scientific, General, Number, Percent, ShortDate, ShortTime, LongDate, LongTime or None. Then you can set Custom Format Strings (http://msdn.microsoft.com/en-us/library/0c899ak8.aspx) to the property CustomFormat.
Here is how to format the YAxis labels:
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