def METHOD_NAME(self):
    x, y, z = list('AB'*10), np.arange(20)%3, np.arange(20)
    ds = Dataset((x, y, z), kdims=['x', 'y'], vdims=['z'],  datatype=[self.datatype])
    keys = [('A', 0), ('B', 1), ('A', 2), ('B', 0), ('A', 1), ('B', 2)]
    grouped = ds.groupby(['x', 'y'])
    self.assertEqual(grouped.keys(), keys)
    group = Dataset({'z': [5, 11, 17]}, vdims=['z'])
    self.assertEqual(grouped.last, group)