如何从熊猫的数据框中绘制直方图

我在pandas中有一个简单的数据框，其中有两个数字列.我想通过熊猫使用matplotlib从列中制作直方图.下面的示例不起作用:

I have a simple dataframe in pandas that has two numeric columns. I want to make a histogram out of the columns using matplotlib through pandas. The example below does not work:

In [6]: pandas.__version__
Out[6]: '0.14.1'

In [7]: df
Out[7]: 
   a   b
0  1  20
1  2  40
2  3  30
3  4  30
4  4   3
5  3   5

In [8]: df.plot(kind="hist")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-4f53176a4683> in <module>()
----> 1 df.plot(kind="hist")

/software/lib/python2.7/site-packages/pandas/tools/plotting.pyc in plot_frame(frame, x, y, subplots, sharex, sharey, use_index, figsize, grid, legend, rot, ax, style, title, xlim, ylim, logx, logy, xticks, yticks, kind, sort_columns, fontsize, secondary_y, **kwds)
   2095         klass = _plot_klass[kind]
   2096     else:
-> 2097         raise ValueError('Invalid chart type given %s' % kind)
   2098 
   2099     if kind in _dataframe_kinds:

ValueError: Invalid chart type given hist

为什么说无效的图表类型?这些列是数字，可以制成直方图.

why does it say invalid chart type? the columns are numeric and can be made into histograms.