You are absolutely right. We are definitely not handling data updates in the best way.
This is something that we will be visiting for the Q1 2010 release and will do our best to better handle data updates. This is a pretty complex task and I cannot give you a 100% promise (that it will be done in time), but we will definitely consider this behavior with a high priority.
Another thing I'd like to do is to mark a series invisible - how can this best be done?
Unfortunately no. This is another item on our backlog. For now you can try hiding the PointMarks and setting the fill of the series to transparent.
the Telerik team