[wildwood]

Well, like the article went into briefly, the I, IV and V chords are going to be the main ones used in a song, especially one from a rock/blues tradition. (a minor, d minor and e minor are the 1, 4, and 5 for a song in a minor key.)

C and a-minor, F and d-minor, and G and e-minor are related chords, and can be interchanged in a standard progression for a different "feel". So it makes sense that e-minor would go to F (V to IV transition) or a-minor (V to I) more than any others. It's interesting that C and d-minor are under-represented in that transition.

It's interesting that the possible chords are all simple triads, with no options for extra tones. A C6 with particular notes in the bass could be interpreted as an a-minor, for example. There's a fair amount of ambiguity there.