Silverlight 4 charting toolkit: how to create a custom series class?

Question

I need to create a custom series class for the Silverlight 4 toolkit (April 2010 drop). There is a tutorial here:

http://themechanicalbride.blogspot.com/2009/03/writing-your-own-silverlight-chart_25.html

that references an older version of the framework. There have been enough changes that this doesn't work.

Can anyone point me to sample code, or a blog entry that described the process?

Thanks.