The third graph in Section 3.1.1 shows that B causes Y, but in the text, the following is stated (tenth paragraph in the same subsection, which is the third paragraph after the third graph):
"It is telling what is happening, and it is telling what is not happening. For instance, notice that
B has no direct effect on the child’s earnings except through its effect on schooling."
Perhaps the mistake is that you used the same graph as the last one in Section 3.1.3.
The third graph in Section 3.1.1 shows that B causes Y, but in the text, the following is stated (tenth paragraph in the same subsection, which is the third paragraph after the third graph): "It is telling what is happening, and it is telling what is not happening. For instance, notice that B has no direct effect on the child’s earnings except through its effect on schooling."
Perhaps the mistake is that you used the same graph as the last one in Section 3.1.3.