This is incredible. When I was first learning Chinese (casually, ~20 years ago), my teacher used some Windows software that drew a diagram of the shape of my pronunciation, so she could illustrate what I was getting wrong in some objective way.

The thing you've built is so good, and I would have loved to have it when I was learning Mandarin.

I tried it with a couple of sentences and it did a good job of identifying which tones were off.

You're probably thinking of Praat, which is still around. Even has the same UI as 20 years ago.