带有类别的 pyplot 条形图

Question

我有以下熊猫数据框：

    x   y   cat
0   1   2   1
1   2   3   1
2   3   4   1
3   1   2   2
4   2   3   2
5   3   8   2

所以我有一些 x-y 值和一个类别。

现在我想制作一个带有每个 x 值（1,2 和 3）的条形图，两个条形显示两个类别的 y 值。这不应该那么复杂，但由于某种原因，这仍然有效：

ax = plt.subplot(111)
ax.bar(df[df["cat"] == 1]["x"]-0.2, df[df["cat"] == 1]["y"],width=0.2,color='b',align='center')
plt.show()

但是当我添加第二类时：

ax = plt.subplot(111)
ax.bar(df[df["cat"] == 1]["x"]-0.2, df[df["cat"] == 1]["y"],width=0.2,color='b',align='center')
ax.bar(df[df["cat"] == 2]["x"]+0.2, df[df["cat"] == 2]["y"],width=0.2,color='g',align='center')
plt.show()

python 开始抱怨一个关键错误：

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-63-2188371c6b12> in <module>()
      1 ax = plt.subplot(111)
      2 ax.bar(df[df["cat"] == 1]["x"]-0.2, df[df["cat"] == 1]["y"],width=0.2,color='b',align='center')
----> 3 ax.bar(df[df["cat"] == 2]["x"]+0.2, df[df["cat"] == 1]["y"],width=0.2,color='b',align='center')
      4 plt.show()

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/matplotlib/__init__.py in inner(ax, *args, **kwargs)
   1890                     warnings.warn(msg % (label_namer, func.__name__),
   1891                                   RuntimeWarning, stacklevel=2)
-> 1892             return func(ax, *args, **kwargs)
   1893         pre_doc = inner.__doc__
   1894         if pre_doc is None:

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/matplotlib/axes/_axes.py in bar(self, left, height, width, bottom, **kwargs)
   2103         if align == 'center':
   2104             if orientation == 'vertical':
-> 2105                 left = [left[i] - width[i] / 2. for i in xrange(len(left))]
   2106             elif orientation == 'horizontal':
   2107                 bottom = [bottom[i] - height[i] / 2.

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/matplotlib/axes/_axes.py in <listcomp>(.0)
   2103         if align == 'center':
   2104             if orientation == 'vertical':
-> 2105                 left = [left[i] - width[i] / 2. for i in xrange(len(left))]
   2106             elif orientation == 'horizontal':
   2107                 bottom = [bottom[i] - height[i] / 2.

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/series.py in __getitem__(self, key)
    601         key = com._apply_if_callable(key, self)
    602         try:
--> 603             result = self.index.get_value(self, key)
    604 
    605             if not is_scalar(result):

/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/indexes/base.py in get_value(self, series, key)
   2167         try:
   2168             return self._engine.get_value(s, k,
-> 2169                                           tz=getattr(series.dtype, 'tz', None))
   2170         except KeyError as e1:
   2171             if len(self) > 0 and self.inferred_type in ['integer', 'boolean']:

pandas/index.pyx in pandas.index.IndexEngine.get_value (pandas/index.c:3557)()

pandas/index.pyx in pandas.index.IndexEngine.get_value (pandas/index.c:3240)()

pandas/index.pyx in pandas.index.IndexEngine.get_loc (pandas/index.c:4279)()

pandas/src/hashtable_class_helper.pxi in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8564)()

pandas/src/hashtable_class_helper.pxi in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8508)()

KeyError: 0

Answer 1

好的，知道了。

ax = plt.subplot(111)
ax.bar( (df[df["cat"] == 1]["x"]-0.2).tolist(), df[df["cat"] == 1]["y"].tolist(),width=0.2,color='b',align='center')
ax.bar( (df[df["cat"] == 2]["x"]+0.2).tolist(), df[df["cat"] == 2]["y"].tolist(),width=0.2,color='g',align='center')
plt.show()

问题是df[df["cat"] == 1]["x"] 不返回一个列表而是一个系列，所以包括索引。显然，这个索引实际上被 pyplot 使用，但它实际上不应该在某种程度上使用它。因为如果我们明确地列出它，它就可以工作：

Answer 2

这也可以作为 pandas 中的单行解决方案：

df.pivot_table(values='y', index='x', columns='cat').plot.bar()

pivot_table 调用将创建一个如下所示的表：

cat  1  2
x        
1    2  2
2    3  3
3    4  8