title: Gtk* backend requires pygtk to be installed
body: <p>From within a virtual environment, trying to load a script which uses <code>matplotlib</code>'s <code>GTKAgg</code> backend, I fail with the following traceback:</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/home/user/.virtualenvs/venv/local/lib/python2.7/site-packages/matplotlib/pyplot.py", line 97, in &lt;module&gt;
      _backend_mod, new_figure_manager, draw_if_interactive, _show = pylab_setup()
    File "/home/user/.virtualenvs/venv/local/lib/python2.7/site-packages/matplotlib/backends/__init__.py", line 25, in pylab_setup
      globals(),locals(),[backend_name])
    File "/home/user/.virtualenvs/venv/local/lib/python2.7/site-packages/matplotlib/backends/backend_gtkagg.py", line 10, in &lt;module&gt;
      from matplotlib.backends.backend_gtk import gtk, FigureManagerGTK, FigureCanvasGTK,\
    File "/home/user/.virtualenvs/venv/local/lib/python2.7/site-packages/matplotlib/backends/backend_gtk.py", line 16, in &lt;module&gt;
      raise ImportError("Gtk* backend requires pygtk to be installed.")
  ImportError: Gtk* backend requires pygtk to be installed.
  </code></pre>
  
  <p>The code which I ran in order to produce that <code>ImportError</code> is as follows:</p>
  
  <pre><code>import matplotlib as mpl
  mpl.use('GTKAgg')
  import matplotlib.pyplot as plt
  </code></pre>
  
  <p>When running the very same code after deactivating my virtual environment, everything goes well.</p>
  
  <p>I assumed this may be due to version differences; indeed, such differences exist on my machine. However, the version in the virtual environment is <strong>newer</strong> (1.2.0 versus 1.1.1rc), so I am not expecting less support.</p>
  
  <p>In case it is not clear: my question is how to allow importing <code>pyplot</code> with <code>GTKAgg</code> backend on a new version of <code>matplotlib</code>, or at least an attempt to understand the reasons for this import failure.</p>