Reproduce:

1. Go to https://turnilo.app/#covid19/
2. Change Visualization with selector to Line Chart, you will see possible adjustemnts
3. Pick any adjustment below first i.e. Add a split on
   Expected result:
   Added split
   What happens:
   Nothing, no split added.

What's interesting, it is impossible to add numeric dimension with "+" menu.

If on Totals Visualization, adding split on works fine