I don't have the time right now to come up with a standalone example but I fear I have somewhere an empty list in my dataset(s). anyway, please improve your data input controls in your functions so that the code does not dive so deeply.

File ..., line 9688, in draw_hist_plot
_figure.savefig(filename, dpi=100)
File "/usr/lib64/python2.7/site-packages/matplotlib/figure.py", line 1370, in savefig
self.canvas.print_figure(_args, *_kwargs)
File "/usr/lib64/python2.7/site-packages/matplotlib/backend_bases.py", line 2096, in print_figure
*_kwargs)
File "/usr/lib64/python2.7/site-packages/matplotlib/backends/backend_agg.py", line 492, in print_png
FigureCanvasAgg.draw(self)
File "/usr/lib64/python2.7/site-packages/matplotlib/backends/backend_agg.py", line 440, in draw
self.figure.draw(self.renderer)
File "/usr/lib64/python2.7/site-packages/matplotlib/artist.py", line 54, in draw_wrapper
draw(artist, renderer, *args, *_kwargs)
File "/usr/lib64/python2.7/site-packages/matplotlib/figure.py", line 940, in draw
self.tight_layout(renderer)
File "/usr/lib64/python2.7/site-packages/matplotlib/figure.py", line 1543, in tight_layout
rect=rect)
File "/usr/lib64/python2.7/site-packages/matplotlib/tight_layout.py", line 352, in get_tight_layout_figure
pad=pad, h_pad=h_pad, w_pad=w_pad)
File "/usr/lib64/python2.7/site-packages/matplotlib/tight_layout.py", line 131, in auto_adjust_subplotpars
fig.transFigure.inverted())
File "/usr/lib64/python2.7/site-packages/matplotlib/transforms.py", line 1645, in inverted
self._inverted = Affine2D(inv(mtx), shorthand_name=shorthand_name)
File "/usr/lib64/python2.7/site-packages/numpy/linalg/linalg.py", line 445, in inv
return wrap(solve(a, identity(a.shape[0], dtype=a.dtype)))
File "/usr/lib64/python2.7/site-packages/numpy/linalg/linalg.py", line 328, in solve
raise LinAlgError('Singular matrix')
numpy.linalg.linalg.LinAlgError: Singular matrix

This is mpl-1.2.1.