Vertical Y-axis scale label for line charts

This adds the option to have a label defining the y-axis on line charts.
The following settings are available: label, font family, font size,
font style, font color

![y axis label screenshot](https://cloud.githubusercontent.com/assets/2054914/4710398/50605b98-58a9-11e4-9a54-cd4f24331d36.png)