Wrong label position

The x-position of the label depends on the backend and the pre-render-time resolution of the figure. The position is correct using the jupyter backend, but incorrect in any other case. The problem arises by an incorrect treatment of the return value of get_window_extent(). The return value was assumed to be in pt, but it is in display units and depends on the pre-render-time resolution. This means a render-time transformation does not work.

The solution is to transform the ATLAS label width explicitly to axes units at pre-render-time.