Add label rotation functionality

@darrenli

This is the ThanxWeb commit: https://github.com/marcferna/thanx-web/commit/5a9384b82f1450cad2fff94c83a5d5eefc6f5f81
Tested both Gruff charts that we have and both work:
![screen shot 2014-01-30 at 3 52 08 pm](https://f.cloud.github.com/assets/1187096/2046684/a9d7ebba-8a09-11e3-855f-40f9660bb643.png)
![screen shot 2014-01-30 at 3 52 33 pm](https://f.cloud.github.com/assets/1187096/2046685/a9d843b2-8a09-11e3-8f54-66908dbc57cb.png)