hist raises AttributeError: 'AxesSubplot' object has no attribute 'ndim' with a one row DataFrame
Hello,

this code works fine:

```
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
bins = np.arange(80, 100 + 2, 1)
df = pd.DataFrame({"Name": ["AAA", "BBB"], "ByCol": [1, 2], "Mark": [85, 89]})
df

   ByCol  Mark Name
0      1    85  AAA
1      2    89  BBB

df["Mark"].hist(by=df["ByCol"], bins=bins)
plt.show()
```

but same code with a one row DataFrame such as

```
df = pd.DataFrame({"Name": ["AAA"], "ByCol": [1], "Mark": [85]})
df

   ByCol  Mark Name
0      1    85  AAA
```

raises:

```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-35-93bcd47d24f2> in <module>()
----> 1 df["Mark"].hist(by=df["Mark"], bins=bins)

//anaconda/lib/python2.7/site-packages/pandas/tools/plotting.pyc in hist_series(self, by, ax, grid, xlabelsize, xrot, ylabelsize, yrot, figsize, bins, **kwds)
   2847                             **kwds)
   2848
-> 2849     if axes.ndim == 1 and len(axes) == 1:
   2850         return axes[0]
   2851     return axes

AttributeError: 'AxesSubplot' object has no attribute 'ndim'
```

Kind regards
