'Parameters
artist : `Artist`, `BboxBase`, or `Transform`
The object to compute the offset from.
ref_coord : length 2 sequence
If `artist` is an `Artist` or `BboxBase`, this values is
the location to of the offset origin in fractions of the
`artist` bounding box.
If `artist` is a transform, the offset origin is the
transform applied to this value.
unit : {\'points, \'pixels\'}
The screen units to use (pixels or points) for the offset
input.'

def __init__(self, artist, ref_coord, unit=u'points'):

    self._artist = artist
    self._ref_coord = ref_coord
    self.set_unit(unit)
