[wildwood]

Why would the II chord follow iii? Can you cite some examples of that?

The only pop/rock use of II that I can think of is as a turn to drive II7 -> V7 -> I.

[HTryan]

Exactly. D Major chord is a V/V in C Major, which has only a couple of common uses in popular music according to the Hooktheory database. V/V -> V7 -> I is one common one. Another is a substitution for ii, like: I -> V/V -> IV -> V, see John mayer, Kelly Clarkson, Kenny Chesney etc.

[Goladus]

I agree, I'd really be interested to see what sort of pattern he's thinking about.

[baddox]

I missed that the graph was only showing songs in C major.