Problem

Currently, AxLine.set_xy1 and set_xy2 take x and y as separate parameters. They should also take them as a singlexy parameter, which is consistent with most other setters and would in particular allow the idiomline.set(xy1=..., xy2=...) (orline.set(xy1=..., slope=...).

Proposed solution

Given that the setters were only introduced only in Matplotlib 3.8, we could even consider just deprecating the split (x, y) form out and ultimately only supporting a single xy tuple (tagging this issue for 3.10 as we probably don't want to wait too long in that case); or we can support both forms with the standard dance present e.g. in Axes.set_xlim.