Bug report

I upgraded matplotlib recently and my boxplot stopped working. I realized it was the keyword positions which was taking in a Int64Index array created by pandas. Using Int64Index.values fixes this, but plot takes a Int64Index array so can't see why it isn't desirable for boxplot or why it was removed/no longer works.

Code for reproduction

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
data = np.random.rand(5,2)
years = pd.date_range('1/1/2000',
                      periods=2, freq=pd.DateOffset(years=1)).year                      
# Does not work
plt.figure()
plt.boxplot(data, positions=years)
# Works
plt.figure()
plt.boxplot(data, positions=years.values)
# Works
plt.figure()
plt.plot(years, data[0,:]) 

Actual outcome

Traceback (most recent call last):

  File "<ipython-input-38-2540b7c8b9eb>", line 6, in <module>
    plt.boxplot(data, positions=years)

  File "/Users/Ray/anaconda/lib/python3.6/site-packages/matplotlib/pyplot.py", line 2729, in boxplot
    zorder=zorder, data=data)

  File "/Users/Ray/anaconda/lib/python3.6/site-packages/matplotlib/__init__.py", line 1717, in inner
    return func(ax, *args, **kwargs)

  File "/Users/Ray/anaconda/lib/python3.6/site-packages/matplotlib/axes/_axes.py", line 3378, in boxplot
    manage_xticks=manage_xticks, zorder=zorder)

  File "/Users/Ray/anaconda/lib/python3.6/site-packages/matplotlib/axes/_axes.py", line 3698, in bxp
    cap_x = np.array([cap_left, cap_right])

ValueError: setting an array element with a sequence.

Expected outcome

Expect to produce boxplot and label the xaxis '2000, 2001'
Unfortunately I cannot remember which version I previously used...

Matplotlib version

• Operating system: macOS
• Matplotlib version: 2.1.1
• Matplotlib backend (print(matplotlib.get_backend())): module://ipykernel.pylab.backend_inline
• Python version: Python 3.6.3 :: Anaconda custom (x86_64)
• Pandas version: 0.21.1

Installation

conda upgrade matplotlib

p.s. don't have much dev experience but happy to have a crack at this if it is desired.