Matplotlib Legend Guide basic examples

Question

I am trying to understand how to use the legend() better, specifically how to use proxy artists. I find the Legend guide to be severely lacking. This post is somewhat similar to this one.

I am using Python 2.7.5 on Windows 7, matplotlib version 1.2.1. I wrote this code which is essentially a combination of the examples in the legend guide:

import matplotlib.patches as mpatches
import matplotlib.lines as mlines
import matplotlib.pyplot as plt

line_up, = plt.plot([1,2,3], label='Line 2')
line_down, = plt.plot([3,2,1], label='Line 1')

blue_line = mlines.Line2D([], [], color='blue', marker='*',
                      markersize=15, label='Blue stars')
red_patch = mpatches.Patch(color='red', label='The red data')
plt.legend([red_patch,blue_line])

plt.show()

The legend label colors are not those assigned in the handles, nor are they a patch and a line with stars. I have tried removing and adding the lines, adding and removing the commas after the lines, etc. Are they any better references for using legend()? Any other tips for using proxy artists? I don't understand why the examples give me totally different results...

Answer 1

The Legend Guide was rewritten to be compatible with matplotlib version 1.4.0 or newer. Your version of matplotlib (1.2.1) is over two years old. Don't be surprised if everything shown in the docs does not work with such an old version.

If you upgrade your version, then using

plt.legend(handles=[red_patch,blue_line])

(as is shown in the Legend Guide) instead of

plt.legend([red_patch,blue_line])

yields

Answer 2

I'm still using matplotlib 1.2.1 so I'll tell you what works for me. I find that if I pass the line objects to legend(), I also have to pass the labels separately. [This is also consistent with the matplotlib documentation on legened()]. I've modified your example slightly to do this, and it seems to work:

import matplotlib.patches as mpatches
import matplotlib.lines as mlines
import matplotlib.pyplot as plt

line_up, = plt.plot([1,2,3], label='Line 2')
line_down, = plt.plot([3,2,1], label='Line 1')

blue_line = mlines.Line2D([], [], color='blue', marker='*',
                  markersize=15, label='Blue stars')
red_patch = mpatches.Patch(color='red', label='The red data')

lines = [blue_line, red_patch]
labels = [line.get_label() for line in lines]
plt.legend(lines, labels)

plt.show()