Thanks for the suggestions. This is more complex than it may sound.

The "outside" area is vertically unbounded. We don't have Polygons that are partially unbounded. That means writing a new type of Artist. The first step in such a direction would be writing such an Artist and creating fill_below() and fill_above() functions. "Fill outside" would be an additional step that needs the intersection handling. I suspect also that it'd better be a dedicated function and not a variant of fill_between.

Thank you for your reply. How about using the y limits of the axes as a boundary? Is there some way to keep track of them if they change?

I think we want a polygon where you can specify each point in a different coordinate system (aka transform) as what you really want for outer edges is a blended (data, axes) coordinate system and for the "inner" points you want a pure-data coordinate system.

https://matplotlib.org/stable/api/_as_gen/matplotlib.patches.ConnectionPatch.html sort of does this, but only for two points

@tacaswell maybe we should phrase this differently. In your proposal we'd need not only a different coordinate system per point, but per coordinate (x in data, y in axes). I'm not convinced we need this generality however. It may be simpler to regard the required Artist more like a line segment, which is extended to an "area" by going vertically/horizontally to +/-infinity.

Hello, @Kevin1212918 @antarapal235 @jasonper0042 @ShakthiAngou and I are planning on working on this issue for a software engineering class.

I'm not convinced we need this generality however

For #22409, @hoffm386 suggested filling the boundary region in a way where the fill would remain on zooming in and out.