Custom Tooltips are not possible in a distribution plot.
I think it would be a nice addition to make custom tooltips available for the distribution plot.
It is already possible for a few visualizations, but not yet for the distribution plot (Qlik Help - Custom Tooltips).
The possibility to add extra measures to the tooltip of the distribution plot would give the visualisation added value in my opinion.
I also received questions/feedback about this from a few customers.