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.

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

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