Is it possible to somehow only show the Tooltip for some DataPoints in a Dataseries, not for every one, e.g. every second DataPoint?
This is an advanced topic and not easy to explain quicjly. But, yes it should be possible.
I recommend to investigate this method:
chart.getConfiguration().getTooltip().setFormatter(formatter String)
You can use some Javascript in the formatter. I remind that doing so have also dangers and it is a bit cumbersome to debug.
With some search I found an example here
http://ttlnews.blogspot.fi/2013/04/vaadin-charts-add-on-showing-custom.html
Perhaps by using these ideas you can find a solution.