unexpected axis ticks in tools.plotting.scatter_matrix
The upper left y axis tick marks in a scatter_matrix appear to have units/values pulled from the (0, 0) histogram counts, instead of the data values that are used for all other axis tick values.

Example:

```
In [1]: import numpy as np

In [2]: import pandas as pd

In [3]: pd.__version__
Out[3]: '0.12.0'

In [4]: df = pd.DataFrame(np.random.randn(100,3), columns=['a', 'b', 'c'])

In [5]: pd.tools.plotting.scatter_matrix(df)
Out[5]: 
array([[<matplotlib.axes.AxesSubplot object at 0x1071d5d90>,
        <matplotlib.axes.AxesSubplot object at 0x1089aa0d0>,
        <matplotlib.axes.AxesSubplot object at 0x1071e36d0>],
       [<matplotlib.axes.AxesSubplot object at 0x10a7d4f90>,
        <matplotlib.axes.AxesSubplot object at 0x10a7fd450>,
        <matplotlib.axes.AxesSubplot object at 0x10a8197d0>],
       [<matplotlib.axes.AxesSubplot object at 0x10a83bf90>,
        <matplotlib.axes.AxesSubplot object at 0x10a84fe50>,
        <matplotlib.axes.AxesSubplot object at 0x10a886310>]], dtype=object)
```

![figure_1](https://f.cloud.github.com/assets/6042395/1699217/0ceb08aa-5f89-11e3-9cf2-0b6f4fed6fee.png)
