What are the leaves on?
The leaves are on the tree.
    src        edge_attr  dst
0     0   to the left of    6
1     0   to the left of    7
2     1  to the right of   11
3     2      surrounding    3
4     2      in front of   12
5     3    surrounded by    2
6     4   to the left of    6
7     4   to the left of    7
8     5           behind   10
9     6  to the right of    0
10    6  to the right of    4
11    7  to the right of    0
12    7  to the right of    4
13   10      in front of    5
14   10  to the right of   11
15   11   to the left of    1
16   11   to the left of   10
17   12           behind    2
18   12               on    5
19   12               on   11