Upstream changed the default backend (checking 1.2 and 1.3 diffs):

-backend      : GTKAgg
+backend      : qt4agg

It could be patched to revert to use GTKAgg (or plain Agg) or
make a requires to python-matplotlib-qt4 in the main package.

Thomas (who did update to 1.3 :-), any comments?

*** Bug 1050002 has been marked as a duplicate of this bug. ***

(In reply to Paulo Andrade from comment #1)
> Thomas (who did update to 1.3 :-), any comments?

GTKAgg will be used now again (without any further comment) :)

*** Bug 1046128 has been marked as a duplicate of this bug. ***

It does not work out of the box for python3, as reported in
feedback to my update (TkAgg works).
How to fix it?

Traceback (most recent call last):
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/backend_gtk.py", line 12, in <module>
    import gobject
ImportError: No module named 'gobject'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.3/site-packages/pylab.py", line 1, in <module>
    from matplotlib.pylab import *
  File "/usr/lib64/python3.3/site-packages/matplotlib/pylab.py", line 269, in <module>
    from matplotlib.pyplot import *
  File "/usr/lib64/python3.3/site-packages/matplotlib/pyplot.py", line 98, in <module>
    _backend_mod, new_figure_manager, draw_if_interactive, _show = pylab_setup()
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/__init__.py", line 28, in pylab_setup
    globals(),locals(),[backend_name],0)
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/backend_gtkagg.py", line 10, in <module>
    from matplotlib.backends.backend_gtk import gtk, FigureManagerGTK, FigureCanvasGTK,\
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/backend_gtk.py", line 16, in <module>
    raise ImportError("Gtk* backend requires pygtk to be installed.")
ImportError: Gtk* backend requires pygtk to be installed.

I'm getting the same error as comment #5.  I'm not even sure how to fix this other than switching back to the Qt4 backend.  I don't see any combination of packages I don't already have installed or could install to fix this problem.

I can confirm the problem on my side as well. After upgrading  python3-matplotlib.x86_64 from version 0:1.3.0-1.fc20 to 0:1.3.1-2.fc20 I get the following output:

>>> import matplotlib
>>> import matplotlib.pyplot
Traceback (most recent call last):
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/backend_gtk.py", line 12, in <module>
    import gobject
ImportError: No module named 'gobject'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.3/site-packages/matplotlib/pyplot.py", line 98, in <module>
    _backend_mod, new_figure_manager, draw_if_interactive, _show = pylab_setup()
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/__init__.py", line 28, in pylab_setup
    globals(),locals(),[backend_name],0)
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/backend_gtkagg.py", line 10, in <module>
    from matplotlib.backends.backend_gtk import gtk, FigureManagerGTK, FigureCanvasGTK,\
  File "/usr/lib64/python3.3/site-packages/matplotlib/backends/backend_gtk.py", line 16, in <module>
    raise ImportError("Gtk* backend requires pygtk to be installed.")
ImportError: Gtk* backend requires pygtk to be installed.
>>> matplotlib.get_backend()
'GTKAgg'
>>> 

Manually choosing 'Agg' as backend does provide a workaround, but is not an option for interactive use.

This was a nice opportunity to test the full list of backends, just out of curiosity. I just tried all options that you see in the error message when you try to set an unknown backend:
 ['pdf', 'gdk', 'CocoaAgg', 'emf', 'WebAgg', 'WX', 'pgf', 'GTK3Agg',
  'TkAgg', 'GTK', 'Qt4Agg', 'GTKCairo', 'GTKAgg', 'ps', 'WXAgg',
  'template', 'agg', 'GTK3Cairo', 'cairo', 'MacOSX', 'svg']

My test results:

pdf: works
gdk: gives error: ImportError: No module named 'gobject'
CocoaAgg: gives error: ImportError: The CococaAgg backend required PyObjC to be installed!
emf: gives error: ImportError: No module named 'matplotlib.backends.backend_emf'
WebAgg: gives error: RuntimeError: The WebAgg backend requires Tornado.
WX: gives error: ImportError: Matplotlib backend_wx and backend_wxagg require wxversion, which was not found.
pgf: gives error: FileNotFoundError: [Errno 2] No such file or directory: 'xelatex'
GTK3Agg: gives error: warnings.warn("The Gtk3Agg backend is not known to work on Python 3.x."); Error in atexit._run_exitfuncs: AttributeError: 'FigureManagerGTK3Agg' object has no attribute 'canvas'
TkAgg: gives error: ImportError: No module named 'matplotlib.backends.backend_tkagg (but works after manually installing python3-matplotlib-tk)
GTK: gives error: ImportError: Gtk* backend requires pygtk to be installed (but no python3 version of pygtk is currently available!)
Qt4Agg: gives error: ImportError: No module named 'matplotlib.backends.backend_qt4agg' (but works after manually installing python3-matplotlib-qt4)
GTKCairo: gives error: ImportError: No module named 'gtk'
GTKAgg: gives error: ImportError: Gtk* backend requires pygtk to be installed.
ps: works
WXAgg: gives error: ImportError: Matplotlib backend_wx and backend_wxagg require wxversion, which was not found.
template: works
agg: works
GTK3Cairo: gives error: Error in atexit._run_exitfuncs: AttributeError: 'FigureManagerGTK3Cairo' object has no attribute 'canvas'
cairo: works
MacOSX: [not available on linux]
svg: works

So concluding: only some "non-GUI backends" are working out of the box.

However, after manually installing python3-matplotlib-qt4 and python3-matplotlib-tk these backends do work.

This I found rather weird, since I thought they where pulled in as dependency, so I checked my yum output again. It says:

Updating:
 python3-matplotlib                                x86_64                      1.3.1-2.fc20                        updates                       30 M
Installing for dependencies:
 python-matplotlib-data                            noarch                      1.3.1-2.fc20                        updates                      2.1 M
 python-matplotlib-data-fonts                      noarch                      1.3.1-2.fc20                        updates                      985 k
Updating for dependencies:
 python-matplotlib                                 x86_64                      1.3.1-2.fc20                        updates                       30 M
 python-matplotlib-doc                             x86_64                      1.3.1-2.fc20                        updates                      106 M
 python-matplotlib-qt4                             x86_64                      1.3.1-2.fc20                        updates                       36 k
 python-matplotlib-tk                              x86_64                      1.3.1-2.fc20                        updates                       74 k
 python-matplotlib-wx                              x86_64                      1.3.1-2.fc20                        updates                       57 k

So this reveils the actual problem: 
python3-matplotlib is pulling in the python2 versions of python-matplotlib-qt4, python-matplotlib-tk and python-matplotlib-wx, which obviously do not provide the needed functionality for the python3 matplotlib.

So something seems confused in the requirements defined on python3-matplotlib (or the handling of these requirements).

python-matplotlib-1.3.1-3.fc20 has been submitted as an update for Fedora 20.
https://admin.fedoraproject.org/updates/python-matplotlib-1.3.1-3.fc20

Package python-matplotlib-1.3.1-3.fc20:
* should fix your issue,
* was pushed to the Fedora 20 testing repository,
* should be available at your local mirror within two days.
Update it with:
# su -c 'yum update --enablerepo=updates-testing python-matplotlib-1.3.1-3.fc20'
as soon as you are able to.
Please go to the following url:
https://admin.fedoraproject.org/updates/FEDORA-2014-2352/python-matplotlib-1.3.1-3.fc20
then log in and leave karma (feedback).

python-matplotlib-1.3.1-3.fc20 has been pushed to the Fedora 20 stable repository.  If problems still persist, please make note of it in this bug report.