Error with bbox_inches='tight' in matplotlib

Question

I use the following piece of code in an ipython notebook to save a bar graph as a .png file:
plt.savefig(filename, bbox_inches='tight')

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.

AssertionError
---> 119 plt.savefig(filename,bbox_inches='tight')

C:\Python27\lib\site-packages\matplotlib\pyplot.pyc in savefig(*args,**kwargs)
---> 472 self.canvas.print_figure(*args,**kwargs)

C:\Python27\lib\site-packages\matplotlib\figure.pyc in savefig(self,*args,**kwargs)
---> 1363 self.canvas.print_figure(*args,**kwargs)

C:\Python27\lib\site-packages\matplotlib\backend_bases.pyc
---> 2054 bbox_inches = self.figure.get_tightbbox(renderer)

C:\Python27\lib\site-packages\matplotlib\figure.pyc in get_tightbbox(self,renderer)
---> 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)
---> 714 assert(len(bboxes))

AssertionError:

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.

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?

Answer 1

This usually means no figures are rendered to the canvas. This also explains why, when you the argument is removed there is no corresponding image! For example:

import pylab
pylab.savefig('test', bbox_inches='tight')

Yields a similar error:

    pylab.savefig('test', bbox_inches='tight')
  File "/usr/lib/pymodules/python2.7/matplotlib/pyplot.py", line 471, in savefig
    return fig.savefig(*args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/figure.py", line 1185, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/backend_bases.py", line 1985, in print_figure
    bbox_inches = self.figure.get_tightbbox(renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/figure.py", line 1326, in get_tightbbox
    _bbox = Bbox.union([b for b in bb if b.width!=0 or b.height!=0])
  File "/usr/lib/pymodules/python2.7/matplotlib/transforms.py", line 675, in union
    assert(len(bboxes))

Answer 2

I had the very same error message. I showed the image via gui and then saved it, which yielded the error. I resolved it by first saving it and only thereafter showing it.