Very simple. Daylight Diagram reflects day-light for selected day.
When you navigate your mouse over the time-bar, the Daylight Diagram is displayed.
Daylight Diagram helps you to orient on the time-bar.
But it is also fun to play with.
You may scroll days back and forth watching the daylight time shrinks or grows.
Use Ctrl-Right/Left keys to enjoy realtime animation or use Calendar.
Also you may compare daylight between different cities.
I recommend to compare Russia/Murmansk and Australia/Sydney
You can make Daylight Diagram displayed always in Options/View/Show Daylight Diagram.
Happy weather watching!
Pasha and Yura