Looks really good. I've felt the need for such an app on more than one occasions. But I'll have to wait for the iOS version I guess (based on the other comment).
Aside: Following your github link led me to discover another one of your music related projects, harmony-explorer which seems quite similar to something I've wanted to build for myself.
Thanks - it's good to know there really is demand for iOS!
Parts of Harmony Explorer are kind of still in the app to this very day, namely the modulo-12 arithmetic for notes. And once support for sounding chords is added, that will also be heavily based on how things were done in Harmony Explorer.
Aside: Following your github link led me to discover another one of your music related projects, harmony-explorer which seems quite similar to something I've wanted to build for myself.