We specifically donot want to usedate2num in these test cases. the fact that it is erroring when handeddatetime objects is precisely the kind of thing we wanted to find out by implementing these tests.

The following patch gets most of the way to fixing it such that you can actually pass datetime objects (though I think it may need some consideration regarding what to do if a non-data transform is provided, or whether "slope" is something that can be meaningfully provided with unitful data):

diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.pyindex 9199f1cafa..524c1ae06a 100644--- a/lib/matplotlib/axes/_axes.py+++ b/lib/matplotlib/axes/_axes.py@@ -919,6 +919,9 @@ class Axes(_AxesBase):                                   self.get_yscale() != 'linear'):             raise TypeError("'slope' cannot be used with non-linear scales")+        xy1 = tuple(self._process_unit_info([("x", xy1[0]), ("y", xy1[1])]))+        if xy2 is not None:+            xy2 = tuple(self._process_unit_info([("x", xy2[0]), ("y", xy2[1])]))         datalim = [xy1] if xy2 is None else [xy1, xy2]         if "transform" in kwargs:             # if a transform is passed (i.e. line points not in data space),

I did the minimal change here to get the point-point version working with the default transform (data space). I do think that the final version would need at least some more.

• passingkwargs in
• Maybe pushing the unit transforming down intomlines.AxLine? (trying to trace how baseLine2D does it, if that does it in the class, I'd probably follow suit forAxLine, but not super clear on that, actually...AxLine inherits fromLine2D)
• some more thought aboutslope may be good... it is already disallowed with non-linear scales, may be best to just disallow it with unitful data or declare it to be in explicitly "unitless data space" (currently it states that it is in 'data coordinates'). I think it may actually work out for at least the case of having the same units on both axes (i.e. the slope is unitless then) or possibly also with full unit libraries, though possibly would require pushing units into theAxLine class for it to work.