我得到ImportError:libpng16.so.16只在virtualenv中（ubuntu14.04，python 2.7.6）

Question

使用时matplotlib.pyplot在虚拟环境中的python解释器中，我得到以下错误：

>>> import matplotlib.pyplot as a

  Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/haz/Envs/DataVizProj/local/lib/python2.7/site-packages/matplotlib/pyplot.py", line 29, in <module>
    import matplotlib.colorbar
  File "/home/haz/Envs/DataVizProj/local/lib/python2.7/site-packages/matplotlib/colorbar.py", line 34, in <module>
    import matplotlib.collections as collections
  File "/home/haz/Envs/DataVizProj/local/lib/python2.7/site-packages/matplotlib/collections.py", line 27, in <module>
    import matplotlib.backend_bases as backend_bases
  File "/home/haz/Envs/DataVizProj/local/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 62, in <module>
    import matplotlib.textpath as textpath
  File "/home/haz/Envs/DataVizProj/local/lib/python2.7/site-packages/matplotlib/textpath.py", line 18, in <module>
    from matplotlib.mathtext import MathTextParser
  File "/home/haz/Envs/DataVizProj/local/lib/python2.7/site-packages/matplotlib/mathtext.py", line 60, in <module>
    import matplotlib._png as _png
ImportError: libpng16.so.16: cannot open shared object file: No such file or directory

我已经为matplotlib和我安装了ZLIB、FREETYPE、LIBPNG 只有在virtualenv中才会出现这个错误。我是一个初学者并且遵循这个tutorial。 顺便说一句，我不使用Python或树冠。只要Python2.7