I'd have to look closer but the graph appears to track AMA's indirect legal wins and losses (AMA itself has never sucsessfully prosecuted anything other than getting 336 passed),
in particular regarding the 3 Taylor vs. FAA cases. The most devistating for AMA was Taylor III in 2018 with the US Court of Appeals rejecting Taylor's claims that the FAA's actions
were arbitrary and capricious with regard to, among other things, that the FAA was unreasonable in declining to define,

"what makes an organization a “nationwide community-based organization,” what qualifies as “programming,” and what activities “endanger the safety of the national airspace.”

The decision refers back to the FAA's interpretation of 336 in 2014:

"In the notice accompanying the Small UAS Rule, the FAA stated that “issues concerning the specific meaning of section 336 (such as what makes an
organization a nationwide community-based organization) are beyond the scope of this rule.” 81 Fed. Reg. at 42,082. The agency explained that it was
“considering the specific meaning of section 336 provisions in a separate regulatory action,” and that, “in order to avoid duplication, [it] limited the scope
of the model-aircraft component of th[e] rulemaking simply to codifying the FAA’s enforcement authority over model-aircraft operations that endanger the
safety” of the national airspace."

"Contrary to Taylor’s complaint, it was not unreasonable (or unconstitutional) for the FAA [in 2014] to use the instant rulemaking merely to codify section
336 and to consider the specific meaning of section 336’s provisions in the separate regulatory action that it had already begun. “An agency enjoys broad
discretion in determining how best to handle related, yet discrete, issues in terms of . . . priorities” and “need not solve every problem before it in the
same proceeding.”

In other words, AMA's belligerence and defiance of the FAA (and forced membership scam) ended on that day and AMA was screwed from that point on -
336 was repealed, the FAA took over, and I'll be interested in seeing that graph in 2 more years.