#60 Allow axes to be places manually

[Achim Westermann]

Most often users want to be able to have axis x and y crossing at (0,0).

Thoughts on this:
A way to implement this would be:

- Allow to specify manual placement for AAxis.
- Those manual places axes would not count in the computation of the axis widths but simply just rendered at the specified value.
- This would allow to have other axes rendered as before while supporting manually places axes.
- Disadvantage: Axes might be placed out of the visible area, axis labels might be overwritten by traces as they stay next to the axis.