x = np.random.randn(50)
plt.plot(x, label=0)
plt.legend()

x = np.random.randn(50)
plt.hist(x, label=0)
plt.legend()

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-27407c59c8b6> in <module>()
      1 x = np.random.randn(50)
----> 2 plt.hist(x, label=0)
      3 plt.legend()

/Users/mwaskom/anaconda/lib/python2.7/site-packages/matplotlib/pyplot.pyc in hist(x, bins, range, normed, weights, cumulative, bottom, histtype, align, orientation, rwidth, log, color, label, stacked, hold, **kwargs)
   2894                       histtype=histtype, align=align, orientation=orientation,
   2895                       rwidth=rwidth, log=log, color=color, label=label,
-> 2896                       stacked=stacked, **kwargs)
   2897         draw_if_interactive()
   2898     finally:

/Users/mwaskom/anaconda/lib/python2.7/site-packages/matplotlib/axes/_axes.pyc in hist(self, x, bins, range, normed, weights, cumulative, bottom, histtype, align, orientation, rwidth, log, color, label, stacked, **kwargs)
   5858         else:
   5859             raise ValueError(
-> 5860                 'invalid label: must be string or sequence of strings')
   5861 
   5862         if len(labels) < nx:

ValueError: invalid label: must be string or sequence of strings