I noticed three backend related issues with matplotlib 3.0.0rc1 on Windows:

1. Setting backend = TkAgg in setup.cfg before building wheels, TkAgg does not get enabled as the default backend in the mpl-data/matplotlibrc file. That option has been removed in 56acb0f#diff-2eeaed663bd0d25b7e608891384b7298. I am aware this is (sort of) mentioned in the whats_new document. TkAgg has always been the default backend in the official binaries for Windows. IMO, this default has been more predictable, reliable (e.g. see issue 2), and faster loading than other backends.

2. At least on my system with PyQt4 installed, the default backend is unusable:

>>> from matplotlib import pyplot
>>> pyplot.plot()
[]
>>> pyplot.show()
Traceback (most recent call last):
  File "X:\Python37\lib\site-packages\matplotlib\backends\backend_qt5agg.py", line 57, in paintEvent
    buf, buf.shape[1], buf.shape[0], QtGui.QImage.Format_RGBA8888)
AttributeError: type object 'QImage' has no attribute 'Format_RGBA8888' 

A window opens but no plot is rendered.

1. I am unable to print the default backend:

>>> import matplotlib
>>> print(matplotlib.get_backend())
<object object at 0x000002928A3FE7A0> 

Matplotlib version

• Operating system: Windows 10 Pro WS 1803
• Matplotlib version: 3.0.0rc1
• Matplotlib backend (print(matplotlib.get_backend())): <object object at 0x000001F814B0E7A0>
• Python version: Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 27 2018, 04:59:51) [MSC v.1914 64 bit (AMD64)] on win32
• Jupyter version (if applicable):
• Other libraries:

Installed from source matplotlib-3.0.0rc1.zip