This is a pretty simple graph. The x-axis is episodes in air order, marked by season, and the y-axis is (number of episodes with a solo)-(number of episodes without a solo) at that x. (Thus, when it goes higher, more solos/long openings are being used. At zero, it's half-and-half.)
I've also marked the greatest differences in both directions as well as all the zeroes. Come up with any interpretations you like.

Back to the solo list (especially if you didn't get here from it!)