We introduce a lifting of West's stack-sorting map $s$ to partition diagrams, which are combinatorial objects indexing bases of partition algebras. Our lifting $\mathscr{S}$ of $s$ is such that $\mathscr{S}$ behaves in the same way as $s$ when restricted to diagram basis elements in the order-$n$ symmetric group algebra as a diagram subalgebra of the partition algebra $\mathscr{P}_{n}^ξ$. We then introduce a lifting of the notion of $1$-stack-sortability, using our lifting of $s$. By direct analogy with Knuth's famous result that a permutation is $1$-stack-sortable if and only if it avoids the pattern $231$, we prove a related pattern-avoidance property for partition diagrams, as opposed to permutations, according to what we refer to as stretch-stack-sortability.