http://jingyan.baidu.com/article/1876c852b9c79d890a137663.html

http://blog.csdn.net/yywan1314520/article/details/50818471

http://blog.csdn.net/czp11210/article/details/51161585

http://matplotlib.org/examples/statistics/histogram_demo_multihist.html

http://matplotlib.org/examples/pylab_examples/histogram_percent_demo.html

hist