plot()是画线图，
scatter()是画散点图
hist()画柱状图：把数值做成一个list,然后调用hist,并制定bins的个数。
show()方法是展示出来
做一个年份与人口的例子：
In [1]: import matplotlib.pyplot as plt
In [2]: year = [1950,1970,1990,2010]
In [3]: pop = [2.519,3.692,5.263,6.972]
In [4]: plt.plot(year,pop)
Out[4]: [<matplotlib.lines.Line2D at 0x7fcea2943e80>]
In [5]: plt.show()
In [6]: plt.scatter(year,pop)
Out[6]: <matplotlib.collections.PathCollection at 0x7fcea295f978>
In [7]: plt.show()
柱状图：
In [2]: import matplotlib.pyplot as plt
In [3]: values=[0,0.3,0.4,0.6,2,3,4.5,4.6,6,4]
In [4]: plt.hist(values,bins=3)
Out[4]:
(array([ 4., 2., 4.]),
array([ 0., 2., 4., 6.]),
<a list of 3 Patch objects>)
In [5]: plt.show()
作图个性化：
