Right now, if I give it I V vi IV, it'll show me "Complicated" by Avril Lavigne. While that song does contain those chords, in that order, they function very differently - because the actual progression is vi IV I V.
Part of the problem is that sometimes when people analyze a section of a song, they include a pick up, etc. Other times the song doesn't have a strong resolution, so it's not clear where a progression begins or ends.
We thought this was the best compromise for now. The tool will show you all songs that start with the progression, as well as songs that use the progression in a phase shifted manner (which is also interesting).
Thanks for the reply. And for making this. I'm a huge fan.