Are there any black numbers or letters?
No, there are letters but they are white.
    src        edge_attr  dst
0     0  to the right of    4
1     0  to the right of    1
2     0  to the right of    8
3     1   to the left of    0
4     1  to the right of    5
5     1   to the left of    8
6     1   to the left of   12
7     1   to the left of   10
8     3   to the left of   12
9     4   to the left of    0
10    5   to the left of    9
11    5   to the left of    7
12    5   to the left of    1
13    7  to the right of    9
14    7  to the right of    5
15    8   to the left of    0
16    8  to the right of    1
17    9   to the left of    7
18    9  to the right of    5
19   10  to the right of    1
20   12  to the right of    3
21   12  to the right of    1