Only two
Andrew Johnson and Bill Clinton. Nixon resigned in order to avoid impeachment, the vote on which was being discussed at the time of his resignation. You're right: chances are he would have been the first to be convicted.
If Tom is tired of the Yeti, maybe you can work with the Loch Ness Monster. There are photos of that, and you could include links to audio files of bagpipe music.
Keep it up.
Regards--
Mary Anne