Connect the founder of the clade we are considering to the Amphioxus branch, not one of the other two. Your cladogram is complete, and, so we believe, correct. A is the outgroup or sister group. Jaws appear within the vertebrates. Legs evolved too within vertebrates, and did so after jaws, or so we believe.
Legs define the Tetrapod clade.
We begin as before. Notice something rather very important. The frog that we added to the first table fit in nicely, neatly, with respect to more primitive characters. I mean that it was like its sister group, the zebrafish, with respect to the cranium and jaws character states. Legs and jaws are derived within vertebrates, not shared by all of them.