I noticed that your question is related to the Kendo UI chart. This is the Silverlight forum and this thread is related to the RadChart for Silverlight. Can you please post your question in new thread in the Kendo forum
However, keep in mind that there is no default format string for such formatting. Instead you can consider using label template
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.