_transform limitations

return newtrans if not units == 'inches': raise

    > ValueError('units must be dots or inches') if fig is None:

This all looks great and I like the interface. My only suggestions is
to add points (1/72. inches) since this is commonly used throughout
matplotlib, is easy, and is the most common distance metric used in

    > Minimal testing and illustration of the use of offsets is
    > now in examples/transoffset.py. It includes cartesian and
    > polar coordinates.