When trying to customize any label in the chart you will want to work with the respective FormatString property whether it is axis labels or tooltips. There are some default format strings you can use that are listed here:
If a default format string will not suffice, then you will have to write a class that implements the IRenderLabel interface. This interface has one method that you are to implement, ToString. This method is passed a hastable that contains information for a given string that is currently being drawn. Here is a more detailed explanation in the help:
http://help.infragistics.com/Help/NetAdvantage/NET/2008.1/CLR2.0/html/Chart_Customize_Labels_Using_the_IRenderLabel_Interface.html
Here is a sample from our Knowledge Base:
http://devcenter.infragistics.com/Support/KnowledgeBaseArticle.aspx?ArticleID=2807
This is a WinForms sample but the same applies for both our win and web charts. Notice that a DataTable that the chart is bound to is passed into the class that implements IRenderLabel. Sometimes this is necessary, if the context hashtable does not contain the information you need to formulate your new string. You can then work off values such as the row and column to figure out what what string you want to return from your DataTable.
Can you please update these links as they no longer work, thank you