Here are some graphs made from the current Linux source code as taken from Linus github's mirror. They show the evolution over versions of Linux (from 2.6.12 to 3.19 excluding all release candidates "-rc[0-9]+") in term of size, number of files, number of lines of codes or authors and commiters. The last one shows that Linux releases are somehow pretty linear. Theses images are licenced CC-BY-SA (Olivier DELHOMME). Upon demand.

Update 2015-04-14 : version 4.0 added, some bugs corrected in my scripts and added a graph on the number of commits.
Update 2015-07-19 : version 4.1 added. Update 2015-09-13 : Version 4.2 added and correctness of linux release dates.

Linux size

Linux files

Linux lines

Linux authors

Linux dates