Is it possible to add a Words Learned graph on the Progress page, just like we have for characters?

That would be encouraging. As time goes on, I've had more and more words added, and fewer characters (as you would expect). It would be nice to see two graphs, so that if the character graph was tailing off a bit, the word graph could give a sense of on-going progress.

我完全同意。

It's on the list, and not very far down. It will probably come along with some other minor progress page upgrades.

Yep, this is one of the things I'll be doing to the progress page in the next few days.