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.

Another graph! The x-axis is as above, and the y-axis marks which solo is used (by number). This is meant to illustrate some trends of frequency and solo introduction. 4-13 (and 44-45) are not in perfect order; there were some ambiguities on episodes' original airings, and I've avoided renumbering them.

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