Getting matplotlib backends for python 3.6 - qt

Question

I installed ipython and matplotlib with pip (9.0.1) under python 3.6 (in Xubuntu 16.04), but no images are showing when I try to plot something.
starting ipython with ipython3 --matplotlib qt gives the following error:
ImportError: Matplotlib qt-based backends require an external PyQt4, PyQt5,
or PySide package to be installed, but it was not found.
I tried to install these with pip, but it fails:
$ pip3.6 install PySide
Collecting PySide
Downloading PySide-1.2.4.tar.gz (9.3MB)
100% |████████████████████████████████| 9.3MB 12.8MB/s
Complete output from command python setup.py egg_info:
only these python versions are supported: [(2, 6), (2, 7), (3, 2), (3, 3), (3, 4)]
----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-bzzpzy5q/PySide/
$ pip3.6 install PyQT
Collecting PyQT
Could not find a version that satisfies the requirement PyQT (from versions: )
No matching distribution found for PyQT
$ pip3.6 install PyQT4
Collecting PyQT4
Could not find a version that satisfies the requirement PyQT4 (from versions: )
No matching distribution found for PyQT4
$ pip3.6 install PyQT5
Collecting PyQT5
Could not find a version that satisfies the requirement PyQT5 (from versions: )
No matching distribution found for PyQT5
If I try ipython3 --matplotlib gtk, the error is:
ImportError: Gtk* backend requires pygtk to be installed.
But:
$ pip3.6 install pygtk
Collecting pygtk
Could not find a version that satisfies the requirement pygtk (from versions: )
No matching distribution found for pygtk
I seem to understand that something called PyGobject or PyGI replaced pygtk for python 3. And indeed, ipython3 --matplotlib gtk3 results in:
ImportError: Gtk3 backend requires pygobject to be installed.
But:
$ pip3.6 install pygobject
Collecting pygobject
Could not find a version that satisfies the requirement pygobject (from versions: )
No matching distribution found for pygobject
$ pip3.6 install PyGObject
Collecting PyGObject
Could not find a version that satisfies the requirement PyGObject (from versions: )
No matching distribution found for PyGObject
Finally, pip3.6 install PyGI succeeds!
But matplotlib still complains about GTK things not being installed.
What else should I try?

Answer 1

Getting the Tk backend to work
As #ImportanceOfBeingErnest suggested, at least one backend should be available: the Tk based one.
This is true provided the Tk development libraries were available when python was compiled. This was not the case for me (I suppose a pre-compiled python distribution should be Tk-enabled).
When I tried to start ipython with Tk as matplotlib backend (ipython3 --matplotlib tk), I had an error message similar to the following one:
import _tkinter # If this fails your Python may not be configured for Tk
ImportError: No module named _tkinter
A comment to the following answer explains how to get the Tk development libraries in Ubuntu: apt install tk-dev.
After doing this and recompiling python 3.6, an ipython3 --matplotlib tk session started without errors and could display graphics.
Setting the default backend choice
The matplotlib documentation gives an example of configuration file, which I downloaded as ~/.config/matplotlib/matplotlibrc. In that file I set backend : TkAgg.
Other backends
The comments in the above-mentioned configuration file mention the existence of yet another GUI backend based on WX, which, like PyGTK and PyQT, doesn't seem to be installable using pip for python 3.6 as of january 2017 (at least in Linux):
$ pip3.6 install wxpython
Collecting wxpython
Could not find a version that satisfies the requirement wxpython (from versions: )
No matching distribution found for wxpython