xenon: add title to graphs; set mindwidth=2

* Add a title to graphs indicating the interval of time they represent.
* Pass --mindwidth=2 to flamegraph.pl to keep the generated SVGs small.
* Typo fix for xenon-log

Change-Id: Ic05bf9876d774218df38736e5785d844059c1a4a
Merge "xenon: add title to graphs; set mindwidth=2" into production