Why can't I see it?
If you have the feature today's usage turned off, you won't see this screen.
Numbers above the graph
Are hours. The dark and light lines under them represent these numbers.
If you turned off an on today's usage
The graph may not reflect these days. If today's usage is turned off, the graph completely ignores these days.
Average sleep lenght
This is approximate. It is that you can compare your usage time with average sleep time.