The LifeLine is the graph at the top of the Timeline page and shows the entire life of the current search. The relative height of its bars (Y-axis) correspond to the relative number of events that occurred in that period of time (X-axis).
The number of events for the life of the current search can be seen in the upper-right:
Entity "well.NO 15/9-F-12" has 5263 lifetime events
In the highlighted area of the image below, you'll see the LifeLine has a gray bar running across it. By default this bar will cover the most-recent 10,000 events.
This gray bar is the TimeSlice Selector and can be adjusted to narrow down the in-scope events that are displayed in the TimeSlice below it.
LifeLine of the entity "well.NO 15/9-F-12"
To adjust the TimeSlice Selector, mouse over the sides of the box until you see the pointer change to double arrows. Click and drag to adjust focus/zoom your view.
Cursor change to double-arrow
The example below shows what this could look like:
The entire area beneath the LifeLine (the TimeSlice) changes based on the TimeSlice Selector