pylab绘图no display name and no $DISPLAY environment variable的解决办法

在stackoverflow中找到了终极解决办法：

>>> import matplotlib
>>> matplotlib.matplotlib_fname()
# This is the file location in Ubuntu
'/etc/matplotlibrc'

找到matplotlibrc之后，将backend从tkAGG修改为AGG

http://stackoverflow.com/questions/2801882/generating-a-png-with-matplotlib-when-display-is-undefined