Set axis background to transparent in inlinebackend

Fixes #5727, related to #4608

Before/after screenshot.

![capture decran 2014-06-29 a 17 35 47](https://cloud.githubusercontent.com/assets/335567/3423658/b10ba754-ffa3-11e3-952c-131d447ca0bf.png)
![capture decran 2014-06-29 a 17 35 53](https://cloud.githubusercontent.com/assets/335567/3423659/b13835b2-ffa3-11e3-8410-b1329034fba0.png)