Bugfix: Avoid exception when returning the index of the point and one or more series are hidden

The last changes regarding the index for the datapoint caused some tests where a series is hidden to fail. This is due to the fact, that the boundaryIds array for the hidden series is skipped. 

This pull-request fixes the issue. However it might be cleaner or better to either have a boundaryIds array without gaps (fill them also if the series is hidden) or don't create the points at all.