matplolib doesn't show legend starting with underscore _

Question

I have the following code in python :

import matplotlib.pyplot as plt
plt.plot([1,2,3],label="_test")
plt.legend()
plt.show()

And when running it, the following warning appears :

/usr/lib/pymodules/python2.7/matplotlib/axes.py:4486: UserWarning: No labeled objects found. Use label='...' kwarg on individual plots.
warnings.warn("No labeled objects found. "

and no legend shows up.

It appears, after trying to remove the '_' at the beginning of the label that the problem comes from here.

Does anybody know why it is so ? Is it a known voluntary/logical feature, an inconvenience or a bug ? Is it reported anywhere, since i couldn't find anything about this on http://matplotlib.org/users/legend_guide.html ? Is it specific to the label of the legend or it occurs as well with some other labels (title, xylabels maybe ?)

Thank you very much for your help

Answer 1

It is documented in the documentation for legend, which is linked from the page you linked to:

Specific lines can be excluded from the automatic legend element selection by defining a label starting with an underscore.