In the UK, we are not taught diagramming, so I can't answer you, I'm afraid. However, does this site help:
- For Teachers
Where does one put a conjunctive adverb in a Reed-Kellogg diagram? Compound sentences joined in this manner also have a semicolon added to the mix. I am tempted to put an (x) to represent the semicolon on the dotted line that joins the independent clauses and then place the conjunctive adverb under the verb of the second independent clause. Any suggestions?