Fix handling scaling for subplots.

Simplified the code so that it works in both cases (subplots and
single figure) by setting xmode resp. ymode.
Fix transformation error

In "_transform_to_data_coordinates" the points are passed to
"Transform.transform" which expects a numpy array. Fix this by
converting points to numpy array.
Merge pull request #44 from hgwd2/master

Bugfixes for subplots and transformation