Adding a legend to a Pandas DataFrame boxplot

Question

I am plotting a series of boxplots on the same axes and want to adda legend to identify them. Very simplified, my script looks like this:

import pandas as pd
import matplotlib.pyplot as plt
import numpy as np
df={}
bp={}
positions = [1,2,3,4]
df[0]= pd.DataFrame (np.random.rand(4,4),columns =['A','B','C','D'])
df[1]= pd.DataFrame (np.random.rand(4,4),columns =['A','B','C','D'])
colour=['red','blue']
fig, ax = plt.subplots()
for i in [0,1]:
    bp[i] = df[i].plot.box(ax=ax,
                          positions = positions,
                          color={'whiskers': colour[i],
                                 'caps': colour[i],
                                 'medians': colour[i],
                                 'boxes': colour[i]}
                          )
plt.legend([bp[i] for i in [0,1]], ['first plot', 'second plot'])
fig.show()

The plot is fine, but the legend is not drawn and I get this warning

    UserWarning: Legend does not support <matplotlib.axes._subplots.AxesSubplot object at 0x000000000A7830F0> instances.
A proxy artist may be used instead.

(I have had this warning before when adding a legend to a scatter plot, but the legend was still drawn, so i could ignore it. )

Here is a link to a description of proxy artists, but it is not clear how to apply this to my script. Any suggestions?

Longwen Ou · Accepted Answer · 2017-10-26 23:21:53Z

'pandas' plots return AxesSubplot objects which can not be used for generating legends. You must generate you own legend using proxy artist instead. I have modified your code:

import pandas as pd
import matplotlib.pyplot as plt
import numpy as np
import matplotlib.patches as mpatches
df={}
bp={}
positions = [1,2,3,4]
df[0]= pd.DataFrame (np.random.rand(4,4),columns =['A','B','C','D'])
df[1]= pd.DataFrame (np.random.rand(4,4),columns =['A','B','C','D'])
colour=['red','blue']
fig, ax = plt.subplots()
for i in [0,1]:
    bp[i] = df[i].plot.box(ax=ax,
                          positions = positions,
                          color={'whiskers': colour[i],
                                 'caps': colour[i],
                                 'medians': colour[i],
                                 'boxes': colour[i]}
                          )

red_patch = mpatches.Patch(color='red', label='The red data')
blue_patch = mpatches.Patch(color='blue', label='The blue data')
plt.legend(handles=[red_patch, blue_patch])

plt.show()

The results are shown below:

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Adding a legend to a Pandas DataFrame boxplot

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