The grouping functionality of the stacked series has already been implemented in Q3 2013 Beta, released yesterday and is illustrated in the online beta demos here - HtmlChart - Stacked Series
This functionality will also be available in the upcoming official Q3 2013, scheduled for the middle of October.
You may also find interesting Q3 2013 BETA (version 2013.3.926) release notes
as well as Check out the Q3 2013 Beta release of RadControls for ASP.NET AJAX
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