Bug report

Bug summary
When trying to show a proportional colorbar with only 3 bounds I get a "division by zero" warning. I don't get the warning if I put 4 bounds, like that: bounds = [0, 3, 5, 7].

The thing is, I need a bar that shows me, for example, that having 5 animals, 3 of them are dogs. Am I using correctly the mpl.colorbar function? Here's the code:

Code for reproduction

import matplotlib.pyplot as plt
import matplotlib as mpl
fig = plt.figure(figsize=(3, 2))
ax1 = fig.add_axes([0.1, 0.5, 0.8, 0.2])
cmap = mpl.colors.ListedColormap(['green', 'red'])
bounds = [0, 3, 5]
cb2 = mpl.colorbar.ColorbarBase(ax1, cmap=cmap, boundaries=bounds,
                                spacing='proportional', orientation='horizontal')
cb2.set_label('3 of 5')
plt.show()

Actual outcome

C:\ProgramData\Miniconda3\lib\site-packages\matplotlib\colorbar.py:810: RuntimeWarning: divide by zero encountered in double_scalars
  automin = (y[2] - y[1]) / clen
C:\ProgramData\Miniconda3\lib\site-packages\matplotlib\colorbar.py:811: RuntimeWarning: divide by zero encountered in double_scalars
  automax = (y[-2] - y[-3]) / clen

Expected outcome
No warning output, I think the code is correct.

Matplotlib version

• Operating System: Windows 10
• Matplotlib Version: 2.0.0 np112py36_0
• Python Version: 3.6.1
• Jupyter Version (if applicable): 5.0.0

Python and Matplotlib installed from conda, default channel.

Thanks in advance!