python - Print string over plotted line (mimic contour plot labels)

Question

The contour plot demo shows how you can plot the curves with the level value plotted over them, see below.

Is there a way to do this same thing for a simple line plot like the one obtained with the code below?

import matplotlib.pyplot as plt 

x = [1.81,1.715,1.78,1.613,1.629,1.714,1.62,1.738,1.495,1.669,1.57,1.877,1.385]
y = [0.924,0.915,0.914,0.91,0.909,0.905,0.905,0.893,0.886,0.881,0.873,0.873,0.844]

# This is the string that should show somewhere over the plotted line.
line_string = 'name of line'

# plotting
plt.plot(x,y)
plt.show()

Answer 1

You could simply add some text (MPL Gallery) like

import matplotlib.pyplot as plt 
import numpy as np
x = [1.81,1.715,1.78,1.613,1.629,1.714,1.62,1.738,1.495,1.669,1.57,1.877,1.385]
y = [0.924,0.915,0.914,0.91,0.909,0.905,0.905,0.893,0.886,0.881,0.873,0.873,0.844]

# This is the string that should show somewhere over the plotted line.
line_string = 'name of line'

# plotting
fig, ax = plt.subplots(1,1)
l, = ax.plot(x,y)
pos = [(x[-2]+x[-1])/2., (y[-2]+y[-1])/2.]
# transform data points to screen space
xscreen = ax.transData.transform(zip(x[-2::],y[-2::]))
rot = np.rad2deg(np.arctan2(*np.abs(np.gradient(xscreen)[0][0][::-1])))
ltex = plt.text(pos[0], pos[1], line_string, size=9, rotation=rot, color = l.get_color(),
     ha="center", va="center",bbox = dict(ec='1',fc='1'))

def updaterot(event):
    """Event to update the rotation of the labels"""
    xs = ax.transData.transform(zip(x[-2::],y[-2::]))
    rot = np.rad2deg(np.arctan2(*np.abs(np.gradient(xs)[0][0][::-1])))
    ltex.set_rotation(rot)

fig.canvas.mpl_connect('button_release_event', updaterot)
plt.show()

which gives

This way you have maximum control.
Note, the rotation is in degrees and in screen not data space.

Update:

As I recently needed automatic label rotations which update on zooming and panning, thus I updated my answer to account for these needs. Now the label rotation is updated on every mouse button release (the draw_event alone was not triggered when zooming). This approach uses matplotlib transformations to link the data and screen space as discussed in this tutorial.