[FIXED JENKINS-22688] Made graph font explicit and configurable

Use of whatever is the default font can make the labels difficult to read. `SansSerif` as explicit default should fix this in most cases, but in case of any weird platform messing up even that, make it configurable. This also allows increasing the font size for better accessibility (especially combined with #1198's large graphs), and matching the font used in the graphs to any custom Jenkins CSS.

![Screenshot](https://cloud.githubusercontent.com/assets/1831569/2749582/e77e19f6-c80e-11e3-9bf8-fbad2cd96692.png)