I'm not entirely sure I like the way zdir works. It uses juggle_axes, so it does like other 2D-converted things and rotates what x/y/z mean. This makes sense for an artist that's a 2D plane, but this is 3D. On the other hand, Line3D works the same way. Perhaps the direction of stem lines/baseline should not be tied to zdir, but use orientation like in 2D?

On further thought, I think zdir may be the wrong approach, at least as used here. Though it was suggested in #6271, I don't think it does what we want. Namely, it decides, which way is 'up' for a 2D-to-3D conversion. But the parameters of stem3d are 3D already. Thus, using zdir rotates the result, and means you need to juggle around x/y/z if have an existing set and just want the stems in a different orientation starting from a different.

If we want to keep zdir, then I think it should apply only to the bottom baseline. Alternatively, we can have orientation={x, y, z}, which moves only the baseline.

After discussing with @tacaswell, I changed this to use orientation instead of zdir, and tweaked the tests a bit.

I am 👍 in general, my primary concern is only being able to control the style via the plot-style format strings. I think that this is OK to start with, but long term we may want to expose all of the knobs.

However, this matches the 2D stem (https://matplotlib.org/api/_as_gen/matplotlib.pyplot.stem.html) and you can always change the styling after the fact.