Hi!
I have a little user experience optimization suggestion. It would be useful if when choosing the chord that you want to see the secondary chords for, instead of having to click on the moving arrow it was a click, hold, and drag ability. So in the picture (hopefully it attaches properly), if I wanted to see the secondary dominant for the V chord (G7) instead of having to click 4 times on the arrows (with the arrows position moving each time I click), I could just click and hold once on the arrow to initiate the move and then release the click to drop the selector at the chord I want to see the dominant for.
This isn’t a huge issue of course, just a small workflow optimization.