I guess what we need is a slightly more general concept of a chunking, which is a sequence of transaction sets, without the requirement that their feerates are monotonically decreasing.
And we can define a diagram for such a general chunking too, it’s just not necessarily convex concave.
Let C\left((s_1,s_2,\ldots,s_n)\right) = (t_1,t_2,\ldots,t_m) be the chunking operation, which just merges adjacent sets whenever the later one is higher feerate than the earlier one.
Then there are some theorems that hold, I think, such as:
- C(S) \geq S
- A \geq B \implies C(A) \geq C(B)