I have a range of songs where I have took an 'unfolded' chromagram computed from a series of short-term windows. Specifically, for one song, I have one vector which contains the amplitudes of the chroma's, and a second vector which contains the corresponding MIDI notes. For now, I can plot a histogram of the amplitudes w.r.t the MIDI notes that gives me a visual idea of the range of notes used.

Here are two songs graphed:

As we can see, the 'classical' example is in my opinion more sparse than its neighbouring graph. I've been trying to figure out a way as to how to measure for something like this, since I need to fold such a feature vector into 1 useful statistic.