I considered making an app for this when my first daughter was born. Especially after she had a few words, it was hard to remember which were new or which she had spoken before.

Enter a word, and a date, which defaults to current date. App will let you know if it's a dupe, and produce plots like the article.

Of course, the idea never progressed beyond my to-do list.

I just put these events on Google Calendar. Easy to do from mobile phone and searchable.