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.)
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 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!)