Doing a quick search for "tooltip" in the WebChart forums yielded this nice post:

http://forums.infragistics.com/forums/p/5216/23656.aspx#23656

Which has lots of helpful links and information.

If you do a quick search for tooltip in the WebChart forum you'll get some good results, here is one:

http://forums.infragistics.com/forums/p/5216/23656.aspx#23656

Here are the links that provide some great information:

Predefined Labels:

http://help.infragistics.com/Help/NetAdvantage/NET/2008.1/CLR2.0/html/Chart_Use_Predefined_and_Custom_Label_Styles.html

Making Custom Labels:

http://help.infragistics.com/Help/NetAdvantage/NET/2008.1/CLR2.0/html/Chart_Customize_Labels_Using_the_IRenderLabel_Interface.html

Knowledge Base Article about Making Custom Labels:

http://devcenter.infragistics.com/Support/KnowledgeBaseArticle.aspx?ArticleID=2807

More info about Predefined Labels:

http://help.infragistics.com/Help/NetAdvantage/NET/2008.1/CLR2.0/html/Chart_Label_Formatting.html

Hope this info helps you solve your problem, and let me know how it goes.

-Paul