import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(2,2,1)
ax.scatter([0.5], [0.5])
fig.savefig('test.jpg', bbox_inches='tight')

Traceback (most recent call last):
  File "tif.py", line 8, in <module>
    fig.savefig('test.jpg', bbox_inches='tight')
  File "/Users/tom/Library/Python/2.6/site-packages/matplotlib/figure.py", line 1162, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/Users/tom/Library/Python/2.6/site-packages/matplotlib/backend_bases.py", line 1943, in print_figure
    **kwargs)
  File "/Users/tom/Library/Python/2.6/site-packages/matplotlib/backend_bases.py", line 1803, in print_jpg
    return image.save(filename_or_obj, **options)
  File "/Users/tom/Library/Python/2.6/site-packages/PIL/Image.py", line 1423, in save
    raise KeyError(ext) # unknown extension
KeyError: ''