title: Error with bbox_inches='tight' in matplotlib
body: <p>I use the following piece of code in an ipython notebook to save a bar graph as a .png file:<br>
  <code>plt.savefig(filename, bbox_inches='tight')</code></p>
  
  <p>It works on my computer and I have tried running the script on another computer. However I get the following error when I try to run it on the other machine.</p>
  
  <pre><code>AssertionError
  ---&gt; 119 plt.savefig(filename,bbox_inches='tight')
  
  C:\Python27\lib\site-packages\matplotlib\pyplot.pyc in savefig(*args,**kwargs)
  ---&gt; 472 self.canvas.print_figure(*args,**kwargs)
  
  C:\Python27\lib\site-packages\matplotlib\figure.pyc in savefig(self,*args,**kwargs)
  ---&gt; 1363 self.canvas.print_figure(*args,**kwargs)
  
  C:\Python27\lib\site-packages\matplotlib\backend_bases.pyc
  ---&gt; 2054 bbox_inches = self.figure.get_tightbbox(renderer)
  
  C:\Python27\lib\site-packages\matplotlib\figure.pyc in get_tightbbox(self,renderer)
  ---&gt; 1496 _bbox = Bbox.union([b for b in bb if b.width!=0 or b.height!=0])
  
  C:\Python27\lib\site-packages\matplotlib\transforms.pyc in union(bboxes)
  ---&gt; 714 assert(len(bboxes))
  
  AssertionError:
  </code></pre>
  
  <p>Removing the bbox_inches='tight' argument seems to resolve the error and saves a file but there is no picture there, only a completely blank .png file.</p>
  
  <p>I have made sure that our versions of python, matplotlib and other packages are all the same. Has anyone come across this before? I'm thinking it could be a bug in matplotlib, but then that would not make sense as it works fine on my computer and we have the same versions. Any ideas or suggestions?</p>