>>> import matplotlib.pyplot as plt
>>> bins = [0, 1, 2, 3]
>>> values = [10, 11, 12]
>>> plt.hist(values, bins=bins, histtype='step')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/matplotlib/pyplot.py", line 2958, in hist
    stacked=stacked, data=data, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/matplotlib/__init__.py", line 1813, in inner
    return func(ax, *args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/matplotlib/axes/_axes.py", line 6209, in hist
    ymin = max(ymin*0.9, minimum) if not input_empty else minimum
UnboundLocalError: local variable 'ymin' referenced before assignment