[pvg]

It's a silly and misleading graph.

http://en.wikipedia.org/wiki/Motion_Picture_Production_Code

Alternatively watch something like Howard Hughes's Hell's Angels (1930)

[mechanical_fish]

It is lots of fun reading through the pre-Code movie synopses:

http://en.wikipedia.org/wiki/Baby_Face_(film)

Sex and violence are not some kind of radical new idea. The shape of the OP's graph is almost certainly caused by the censors of the mid-20th century.