After

line = plt.axvline(
    x=.5, ymin=matplotlib.dates.num2date(1), ymax=matplotlib.dates.num2date(1.05))
line.set_clip_on(False)

and panning the axes around, one sees that matplotlib (3.0) converted ymin and ymax to the scalar values 1 and 1.05, which is not correct: ymin and ymax are values relative to the axes bounds and thus intrinsically unitless; passing unitized data to ymin or ymax should result in an error.

The same point likely applies to axhline, axvspan, axhspan.