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

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 (e.g. 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()