Add an annotation (label) to a matplotlib line. Position and Alignment is adjusted to match the line.

label_line.py

def label_line(ax, line, label, color='0.5', fs=14, halign='left'):
    """Add an annotation to the given line with appropriate placement and rotation.

    Based on code from:
        [How to rotate matplotlib annotation to match a line?]
        (http://stackoverflow.com/a/18800233/230468)
        User: [Adam](http://stackoverflow.com/users/321772/adam)

    Arguments
    ---------
    ax : `matplotlib.axes.Axes` object
        Axes on which the label should be added.
    line : `matplotlib.lines.Line2D` object
        Line which is being labeled.
    label : str
        Text which should be drawn as the label.
    ...

    Returns
    -------
    text : `matplotlib.text.Text` object

    """
    xdata, ydata = line.get_data()
    x1 = xdata[0]
    x2 = xdata[-1]
    y1 = ydata[0]
    y2 = ydata[-1]

    if halign.startswith('l'):
        xx = x1
        halign = 'left'
    elif halign.startswith('r'):
        xx = x2
        halign = 'right'
    elif halign.startswith('c'):
        xx = 0.5*(x1 + x2)
        halign = 'center'
    else:
        raise ValueError("Unrecogznied `halign` = '{}'.".format(halign))

    yy = np.interp(xx, xdata, ydata)

    ylim = ax.get_ylim()
    # xytext = (10, 10)
    xytext = (0, 0)
    text = ax.annotate(label, xy=(xx, yy), xytext=xytext, textcoords='offset points',
                       size=fs, color=color, zorder=1,
                       horizontalalignment=halign, verticalalignment='center_baseline')

    sp1 = ax.transData.transform_point((x1, y1))
    sp2 = ax.transData.transform_point((x2, y2))

    rise = (sp2[1] - sp1[1])
    run = (sp2[0] - sp1[0])

    slope_degrees = np.degrees(np.arctan2(rise, run))
    text.set_rotation_mode('anchor')
    text.set_rotation(slope_degrees)
    ax.set_ylim(ylim)
    return text

_Note_: this method needs to be called after the figure layout is finalized, otherwise things will be altered.

import numpy as np
import matplotlib.pyplot as plt

...
fig, axes = plt.subplots()
color = 'blue'
line, = axes.plot(xdata, ydata, '--', color=color)
...
label_line(line, "Some Label", x, y, color=color)

This function is great! Just to suggest an edit though, it would be good to check if the axis is a log axis, e.g., from line 36 change:

elif halign.startswith('c'):        
    xx = 0.5*(x1 + x2)
    halign = 'center'

to:

elif halign.startswith('c'):        
    if ax.get_xscale() == 'log':
        xx = 10**(0.5*(np.log10(x1) + np.log10(x2))
    else:
        xx = 0.5*(x1 + x2)
    halign = 'center'

otherwise the positioning is wrong. And also for line 42 change:

yy = np.interp(xx, xdata, ydata)

to

if ax.get_xscale() == 'log' and ax.get_yscale() == 'log':
    yy = 10**(np.interp(np.log10(xx), np.log10(xdata), np.log10(ydata)))
elif ax.get_xscale() == 'log' and ax.get_yscale() != 'log':
    yy = np.interp(np.log10(xx), np.log10(xdata), ydata)
else:
    yy = np.interp(xx, xdata, ydata)