GP mentioned flowcharts in the pedagogy of medicine which is an easily verifiable statement. Anyone willing to page through Harrison's Principles of Internal Medicine will be able to identify the decision tree figures. They are depicted on a pale green field.

Not the first in the tome, but pg 240[2] contains a great example of such a decision tree.

1. https://archive.org/details/harrisons-principles-of-internal...

2. https://archive.org/details/harrisons-principles-of-internal...