feature: configurable vertical segments

Add ability to create vertical grid.
Screenshot of resulting chart:
[![Screenshot](https://drive.google.com/uc?export=view&id=0B65dEGRmB3ViaHA5U2JERHFmYkE)](https://docs.google.com/file/d/0B65dEGRmB3ViaHA5U2JERHFmYkE/edit)