与熊猫的箱形图：nrows x ncols缺少一个情节

Question

我无法弄清楚这里有什么问题！ 基本上，我正在使用Pandas DataFrame方法df.boxplot绘制2x2箱图，如下所示：

fig, axes = plt.subplots(nrows=2, ncols=2, figsize=(18, 16))
axes = axes.reshape(-1)
for i, crp in enumerate(["Cabbage", "Green beans", "Potato", "Wheat"]):
    print i, crp
    select = pd.Series(crp)
    df[df.Crop.isin(select)].boxplot(
        "Total_Irrigation_mm", by="Climate", ax=axes[i])

我在轴上重复绘制白菜，青豆，马铃薯和小麦的情节。它适用于三种作物然后我收到以下错误：

keys, values = zip(*gp_col)
ValueError: need more than 0 values to unpack and the following output

我的数据框，df是：

     level_0  index       Crop       Climate               Year   Total_Irrigation_mm  
0          0      0       Cabbage       Humid continental  1981   248
1          1      1       Cabbage       Humid continental  1982   257 
2          2      2       Cabbage       Humid continental  1983   329
3          3      3       Green beans   Humid continental  1981   349
4          4      4       Green beans   Humid continental  1982   329
5          5      5       Green beans   Humid continental  1983   303
6          6      6       Potato        Humid continental  1981   278
7          7      7       Potato        Humid continental  1982   393
8          8      8       Potato        Humid continental  1983   393
9          9      9       Wheat         Humid continental  1981   393
10        10     10       Wheat         Humid continental  1982   393
11        11     11       Wheat         Humid continental  1983   393
12        12     12       Cabbage       Temperate          1981   393
13        13     13       Cabbage       Temperate          1982   393
14        14     14       Cabbage       Temperate          1983   393
15        15     15       Green beans   Temperate          1981   393
16        16     16       Green beans   Temperate          1982   393
17        17     17       Green beans   Temperate          1983   393
18        18     18       Potato        Temperate          1981   393
19        19     19       Potato        Temperate          1982   393
20        20     20       Potato        Temperate          1983   318
21        21     21       Wheat         Temperate          1981   312
22        22     22       Wheat         Temperate          1982   350
23        23     23       Wheat         Temperate          1983   152
24        24     24       Cabbage       Subtropical        1981   159  
25        25     25       Cabbage       Subtropical        1982   157 
26        26     26       Cabbage       Subtropical        1983   122
27        27     27       Green beans   Subtropical        1981   196
28        28     28       Green beans   Subtropical        1982   126   
29        29     29       Green beans   Subtropical        1983   188  
30        30     30       Potato        Subtropical        1981   123
31        31     31       Potato        Subtropical        1982   113 
32        32     32       Potato        Subtropical        1983   211
33        33     33       Wheat         Subtropical        1981   152  
34        34     34       Wheat         Subtropical        1982   158 
35        35     35       Wheat         Subtropical        1983   158



Traceback (most recent call last):
  File "C:\Users\*\Anaconda\lib\site-packages\IPython\core\interactiveshell.py", line 2883, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-1071-c19c414988ec>", line 7, in <module>
    "Total_Irrigation_mm", by="Climate", ax=axes[i])
  File "C:\Users\*\Anaconda\lib\site-packages\pandas\core\frame.py", line 4973, in boxplot
    **kwds)
  File "C:\Users\*\Anaconda\lib\site-packages\pandas\tools\plotting.py", line 2368, in boxplot
    ax=ax, layout=layout, return_type=return_type)
  File "C:\Users\*\Anaconda\lib\site-packages\pandas\tools\plotting.py", line 2763, in _grouped_plot_by_column
    keys, values = zip(*gp_col)
ValueError: need more than 0 values to unpack