Add a label to y-axis to show the value of y for a horizontal line in matplotlib

Question

How can I add a string label to the horizontal red line showed in the following plot? I want to add something like "k=305" to the y-axis label next to the line. The blue dots are just some other data and the values do not matter. For recreation of this problem, you can plot any kind of data. My question is about the red line.

plt.plot((0,502),(305,305),'r-')
plt.title("ALS+REG")

Answer 1

A horizontal line can be drawn using Axes.axhline(y) .
Adding a label would be done by using Axes.text() . THe tricky bit is to decide upon the coordinates at which to place that text. Since the y coordinate should be the data coordinate at which the line is drawn, but the x coordinate of the label should be the independent of the data (eg allow for differen axis scales), we can use a blended transform, where the x transform is the transform of the axis ylabels, and the y transform is the data coordinate system.

import matplotlib.pyplot as plt
import matplotlib.transforms as transforms
import numpy as np; np.random.seed(42)

N = 120
x = np.random.rand(N)
y = np.abs(np.random.normal(size=N))*1000
mean= np.mean(y)

fig, ax=plt.subplots()
ax.plot(x,y, ls="", marker="o", markersize=2)
ax.axhline(y=mean, color="red")

trans = transforms.blended_transform_factory(
    ax.get_yticklabels()[0].get_transform(), ax.transData)
ax.text(0,mean, "{:.0f}".format(mean), color="red", transform=trans, 
        ha="right", va="center")

plt.show()