DataFrame.plot() does not treat float index as numeric
DataFrame.plot() does not treat float index as numeric.

This is becasue `DataFrame.index.is_numeric()` returns `False` when the index holds floats. 
`DataFrame.index.is_numeric()` checks on the dtype of the index to know if it holds numeric data. Apparently dtype of a float index is `object`.

example from the [mailing list](https://groups.google.com/forum/?fromgroups#!topic/pystatsmodels/M77RuKGvC6c)

``` python
In [89]: df = pd.DataFrame(np.random.randn(4,3), columns=['A', 'B',
'C'])

In [90]: df
Out[90]:
         A         B         C
0 -1.209472 -2.016404  0.748993
1  0.568799  1.332006  1.228565
2 -1.105552  0.247344  0.379427
3  0.518771  1.155601  0.441952

In [91]: df.set_index('A')
Out[91]:
                 B         C
A
-1.209472 -2.016404  0.748993
 0.568799  1.332006  1.228565
-1.105552  0.247344  0.379427
 0.518771  1.155601  0.441952

In [93]: df2 = df.set_index('A')

In [98]: df2.plot()
Out[98]: <matplotlib.axes.AxesSubplot object at 0x5a942d0>

In [100]: xlim()
Out[100]: (0.0, 3.0)
```
