Are there numbers or letters that are not black?
No, there are letters but they are black.
     src        edge_attr  dst
0      0   to the left of   27
1      0  to the right of   22
2      0  to the right of   21
3      0  to the right of    1
4      0   to the left of   19
..   ...              ...  ...
140   27  to the right of   22
141   27   to the left of   23
142   27  to the right of    0
143   27  to the right of    9
144   27  to the right of    8

[145 rows x 3 columns]