Documentation here says:
Note that the first event, Send Request (Calendar.html) is shown with a bar that consists
of three shades of blue. The darkest one marks the time that the event itself took, the next one corresponds to the CPU time that this event along with all the nested events
took (here, it includes Receive Response, Receive Data and so on), and the palest bar
stands for the wall time between the start of the first event and the end of the last event.
What exactly is meant by the start of the first event and the end of the last event
?