'Set the dash style for the gc.
Parameters
dash_offset : float
is the offset (usually 0).
dash_list : array_like
specifies the on-off sequence as points.
``(None, None)`` specifies a solid line'

def set_dashes(self, dash_offset, dash_list):

    if (dash_list is not None):
        dl = np.asarray(dash_list)
        if np.any((dl < 0.0)):
            raise ValueError(u'All    values    in    the    dash    list    must    be    positive')
    self._dashes = (dash_offset, dash_list)
