kde plots produce unexpected behavior with integers
If we feed a series of integers into the kde plot the results are a horizontal line:

```
newy = pandas.Series([10,26,36,64,69,73,76,79,80,81,81,82,83,86,89,93,98,126,136,152])
newy.plot(kind='kde')
```

However, if the series is floats, instead of ints, things work as expected:

```
newy = pandas.Series([10.0,26,36,64,69,73,76,79,80,81,81,82,83,86,89,93,98,126,136,152])
newy.plot(kind='kde')
```

I originally noticed the behavior using `scatter_matrix(..., diagonal='kde')` which uses the same plotting engine, I presume. 
