Can you document how this interacts with semilog and loglog plots? (Even if it is "it doesn't work with them".)

Hmm, I can't reproduce the test failure

You can download the results from AppVeyor:

Wrong version of FreeType used for test?

@anntzer I've tried this code, it seems not support any type of log scale yet.

🎉 it works!

what happens if we make it non-linear scale after calling axline?

Then you end up with a line that's incorrect in the same way as if it were set beforehand.

Can we refactor axhline and axvline as special cases of this function?

I also think we should add a what's new entry, and update the following example with this new function: https://matplotlib.org/devdocs/gallery/subplots_axes_and_figures/axhspan_demo.html#sphx-glr-gallery-subplots-axes-and-figures-axhspan-demo-py

thanks!

Reading axhline's and axvline's code, I see that it's not trivial (I do have to say that it's a bit annoying to have such closely related functions with slightly different APIs)

Okay I've done a big 'ol rebase and squash, and this now uses xy1 and xy2.

I think that's all the comments taken care of, so feel free to review again. I've squashed everything into one commit, so there's no extra test image files hanging around any more.

I just noticed another point: Currently, axline() does not trigger an update of the autolimits in use (for example a call to axline() when the view limits are x=0..1, y=0..1 leaves them unchanged); however, adding more plots after a call to axline() shows that the internal data limits have been changed by axline to include x=0..1, y=(whatever y range corresponds to x=0..1 in the axline). For example, after plt.gca().axline((3, 7), (5, 6)); plt.plot([0, 1]), one gets

I think reasonable approaches would be to use the two points passed to axline() as "autoscaling limits", or to just not update the autoscale limits at all (the relevant code appears to be in Axes._update_line_limits, which is called by Axes.add_line).

I agree that not setting auto-limits is a good idea, but after a bit of a play around I'm not sure what the right way to do that is. Is there an easy way to exclude an Artist from autoscale calculations?

This was discussed a while ago and there isn't really a smart way to do it; what I personally do is

@contextmanager
def ignore_in_autoscale(ax):
    """
    When autoscaling, ignore the extents of artists created in the context.
    """
    dl = ax.dataLim.frozen()
    try:
        yield
    finally:
        ax.dataLim = dl
        ax.autoscale_view(scalex=ax.get_autoscalex_on(),
                          scaley=ax.get_autoscaley_on())

(basically, save the datalimits first, add the artist, then restore the datalimits), which is a bit of a hack.

Replaced by #15330.