Shrinking window can break the y dimension scaling

While hammering like crazy on the resizing of the window in your branch, I was able to "break" it 3 times (see screenshot) though I can't seem to find the succession of steps to reproduce it reliably. It seems to appear when I would reduce the window it its smallest possible size, and reduce the vertical length to ~0.


One of the times I broke it that way, I got this:

```
/Users/jrocher/Projects/ETS_trunk/chaco/chaco/axis.py:619: RuntimeWarning: invalid value encountered in divide
self._axis_pixel_vector = self._axis_vector/sqrt(dot(self._axis_vector,self._axis_vector))
```