Show vertical line when hovering graphs, not points

[nhoening]

This should make it much easier to tell the time step currently being hovered.

This is probably a vega-lite/Altair issue to sort out, maybe just one setting, hopefully easy to find. If not, this issue might not be worth it.

[nhoening]

@Flix6x you are our vega-lite/Altair expert. Do you have a hint on how to approach this? Even the decision to look into the Altair vs the vega-lite side if things for this first?

[Flix6x]

The vega-lite specs in belief_charts.py is where this would need to happen. Each subchart would get a ruler with the same condition as the circle marks that show up now already.