Draw a line with matplotlib using the axis coordinate system

Question

I want to plot a vertical line that spans the entire y axis located for example in the x=.25 position of the x axis , not the data axis .

According to this answer (which is apparently not entirely accurate) the axhline,axvline functions would draw a horizontal/vertical line in the axes coordinates , as shown in:

The method axhline and axvline are used to draw lines at the axes coordinate

But this does not seem to work. The axhline docs say :

y position in data coordinates of the horizontal line.

and sure enough, the code given in the answer above displays:

Compare with the old plot shown in the mentioned answer (code below):

Did this change recently or am I missing something very obvious? If it did change, how would I draw a line in the axes coordinate now?

I'm using Python 3.7.3 and matplotlib 3.1.0.

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import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 5, 100)
y = np.sin(x)

fig, ax = plt.subplots()

ax.plot(x, y)
ax.axhline(y=0.5, xmin=0.0, xmax=1.0, color='r')
ax.hlines(y=0.6, xmin=0.0, xmax=1.0, color='b')

plt.show()

Answer 1

Answering the question

If I wanted to plot a vertical line that spanned the entire y axis located in the x=.25 position of the x axis, not the data axis. How would I do that?

In that case both of the x coordinates of that line are 0.25 and the y coordinates are 0 for the lower end and 1 for the upper end. The transform of the line is set to the axes coordinate system ax.transAxes .

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set(xlim=(0.5,1.5), ylim=(-50,50))

ax.plot([0.25,0.25],[0,1], transform=ax.transAxes)

plt.show()