>>> x.sort_index()
2       1
5       2
6       3
8       4

>>> z.sort_index()
1       0
2       1
3       0
4       0
5       2
6       3
7       0
8       4

>>> y = pd.Series([0 for i in range(0,8)])
0    0
1    0
2    0
3    0
4    0
5    0
6    0
7    0

>>> pd.concat([x,z],axis=0)
2    1
5    2
6    3
8    4
0    0
1    0
2    0
3    0
4    0
5    0
6    0
7    0

>>> pd.concat([x,z],axis=1)
    0   1
0 NaN   0
1 NaN   0
2   1   0
3 NaN   0
4 NaN   0
5   2   0
6   3   0
7 NaN   0
8   4 NaN