matplotlib.use('pdf') + plotting png

Traceback (most recent call last):
  File "/home/ant6/INZ/modulators/src/findworker.py", line 411, in run
    self.plot_functions()
  File "/home/ant6/INZ/modulators/src/findworker.py", line 464, in plot_functions
    show=common.check_option('show_sobp_plot'))
  File "/home/ant6/INZ/modulators/src/plotting.py", line 122, in plot_SOBP
    pylab.savefig(filename + '.' + plot_format, format=plot_format)
  File "/usr/lib/python3/dist-packages/matplotlib/pyplot.py", line 561, in savefig
    return fig.savefig(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1422, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/backend_bases.py", line 2129, in print_figure
    print_method = self._get_print_method(format)
  File "/usr/lib/python3/dist-packages/matplotlib/backend_bases.py", line 2070, in _get_print_method
    '%s.' % (format, ', '.join(formats)))
ValueError: Format "png" is not supported.
Supported formats: pdf.
Qt has caught an exception thrown from an event handler. Throwing
exceptions from an event handler is not supported in Qt. You must
reimplement QApplication::notify() and catch all exceptions there.

Podczas instalacji matplotliba:

OPTIONAL BACKEND EXTENSIONS
macosx: no [Mac OS-X only]
qt5agg: yes [installing, Qt: 5.2.1, PyQt: 5.2.1]
qt4agg: no [PyQt4 not found]
pyside: no [PySide not found]
gtk3agg: yes [installing, version 3.8.10]
gtk3cairo: yes [installing, version 3.8.10]
gtkagg: no [Requires pygtk]
tkagg: no [The C/C++ header for Tk (tk.h) could not be found. You may need to install the development package.]
wxagg: no [requires wxPython]
gtk: no [Requires pygtk]
agg: yes [installing]
cairo: yes [installing, pycairo version 1.10.0]
windowing: no [Microsoft Windows only]