import numpy as np
    import matplotlib.pyplot as plt
    
    mrk_array = np.array([[-0.5, 0],
                          [0.5, 0]])

    plt.plot(0, 0, marker=mrk_array)

/Library/Python/2.7/site-packages/matplotlib/markers.pyc in set_marker(self, marker)
    233     def set_marker(self, marker):
    234         if (iterable(marker) and len(marker) in (2, 3) and
--> 235                 marker[1] in (0, 1, 2, 3)):
    236             self._marker_function = self._set_tuple_marker
    237         elif isinstance(marker, np.ndarray):

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()