Q:

How can I set the 'backend' in matplotlib in Python?

I am new user of matplotlib, my platform is Ubuntu 10.04 Python 2.6.5
This is my code
import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt 
plt.plot([1,2,3])

The error is:
/usr/local/lib/python2.6/dist-packages/matplotlib/backends/__init__.py:41: UserWarning: 
Your currently selected backend, 'agg' does not support show().
Please select a GUI backend in your matplotlibrc file ('/usr/local/lib/python2.6/dist-packages/matplotlib/mpl-data/matplotlibrc')
or with matplotlib.use()
  (backend, matplotlib.matplotlib_fname()))

I installed the Anti-Grain Geometry library apt-get install libagg but it is doesn't work.
I tried to use other argument of backend like 'GTK' and 'TkAgg'.
I installed python-gtk2-dev package, but still the error is below.
Can anyone tell me an executable backend argument and its dependency library?

Here is the error:
>>> matplotlib.use('GTK')
>>> import matplotlib.pyplot as plt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.6/dist-packages/matplotlib/pyplot.py", line 95, in <module>
    new_figure_manager, draw_if_interactive, show = pylab_setup()
  File "/usr/local/lib/python2.6/dist-packages/matplotlib/backends/__init__.py", line 25, in pylab_setup
    globals(),locals(),[backend_name])
  File "/usr/local/lib/python2.6/dist-packages/matplotlib/backends/backend_gtk.py", line 28, in <module>
    from matplotlib.backends.backend_gdk import RendererGDK, FigureCanvasGDK 
  File "/usr/local/lib/python2.6/dist-packages/matplotlib/backends/backend_gdk.py", line 29, in <module>
    from matplotlib.backends._backend_gdk import pixbuf_get_pixels_array
ImportError: No module named _backend_gdk

A:

Your currently selected backend, 'agg' does not support show().

AGG backend is for writing to file, not for rendering in a window. See the backend FAQ at the matplotlib web site.

ImportError: No module named _backend_gdk

For the second error, maybe your matplotlib distribution is not compiled with GTK support, or you miss the PyGTK package. Try to install it.
Do you call the show() method inside a terminal or application that has access to a graphical environment?
Try other GUI backends, in this order:

TkAgg
WX
QTAgg
QT4Agg

A:

FYI, I found I needed to put matplotlib.use('Agg') first in Python import order. For what I was doing (unit testing needed to be headless) that meant putting
import matplotlib
matplotlib.use('Agg')

at the top of my master test script. I didn't have to touch any other files.

A:

This can also be set in the configuration file matplotlibrc (as explained in the error message), for instance:
# The default backend; one of GTK GTKAgg GTKCairo GTK3Agg GTK3Cairo
# CocoaAgg MacOSX Qt4Agg Qt5Agg TkAgg WX WXAgg Agg Cairo GDK PS PDF SVG
backend : Agg

That way, the backend does not need to be hardcoded if the code is shared with other people.
For more information, check the documentation.

