I've not been able to figure out a way to show "hidden" data points. I've had to do something similar in the past, and used a custom tooltip template function. In the function I grab the category (x value in my scatter chart sample), and get all matching items from the datasource. Then just put together some sort of list/etc. and return that. You can always go super fancy and make a box with tabs and the whole nine yards to show each dataItem on that particular point.
See sample on jsbin