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.

Answer 1

Based on Jakob's code, here is a function that rotates the text in data space, puts labels near a given x or y data coordinate, and works also with log plots.

def label_line(line, label_text, near_i=None, near_x=None, near_y=None, rotation_offset=0, offset=(0,0)):
    """call 
        l, = plt.loglog(x, y)
        label_line(l, "text", near_x=0.32)
    """
    def put_label(i):
        """put label at given index"""
        i = min(i, len(x)-2)
        dx = sx[i+1] - sx[i]
        dy = sy[i+1] - sy[i]
        rotation = np.rad2deg(math.atan2(dy, dx)) + rotation_offset
        pos = [(x[i] + x[i+1])/2. + offset[0], (y[i] + y[i+1])/2 + offset[1]]
        plt.text(pos[0], pos[1], label_text, size=9, rotation=rotation, color = line.get_color(),
        ha="center", va="center", bbox = dict(ec='1',fc='1'))

    x = line.get_xdata()
    y = line.get_ydata()
    ax = line.get_axes()
    if ax.get_xscale() == 'log':
        sx = np.log10(x)    # screen space
    else:
        sx = x
    if ax.get_yscale() == 'log':
        sy = np.log10(y)
    else:
        sy = y

    # find index
    if near_i is not None:
        i = near_i
        if i < 0: # sanitize negative i
            i = len(x) + i
        put_label(i)
    elif near_x is not None:
        for i in range(len(x)-2):
            if (x[i] < near_x and x[i+1] >= near_x) or (x[i+1] < near_x and x[i] >= near_x):
                put_label(i)
    elif near_y is not None:
        for i in range(len(y)-2):
            if (y[i] < near_y and y[i+1] >= near_y) or (y[i+1] < near_y and y[i] >= near_y):
                put_label(i)
    else:
        raise ValueError("Need one of near_i, near_x, near_y")

Answer 2

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.