If the ASCII character 95 _ is used in as first character in a plot label, the label will not show up in the legend.

If only labels with _ in front are used, the following warning is given:

/opt/pkg/devel/python/3.4.3/lib/python3.4/site-packages/matplotlib-1.4.3-py3.4-linux-x86_64.egg/matplotlib/axes/_axes.py:475: UserWarning: No labelled objects found. Use label='...' kwarg on individual plots.
  warnings.warn("No labelled objects found. "

In order to test this bug, I tried a variety of matplotlib versions on Ubuntu and OS X.

• 0.99.1.1
• 1.1.0
• 1.3.1
• 1.4.3

All have the above behavior.

In order to reproduce the bug, I used the following minimal example:

import numpy as np
import matplotlib
import matplotlib.pyplot as plt

print(matplotlib.__version__)

x = np.linspace(-10, 10, 1024)
y = np.sin(x)

plt.plot(x,y, label="_abc")

plt.legend()
plt.show()

Using r"_abc" instead does not solve the problem.