Stacked bar plot negative values do not work correctly if dataframe contains NaN values
While trying to produce a stacked bar plot which includes negative values, I found that if the dataframe contains NaN values the bar plot does not display correctly.

Specifically, this code:

``` python
df = pd.DataFrame([[10,20,5,40],[-5,5,20,30],[np.nan,-10,-10,20],[10,20,20,-40]], columns = ['A','B','C','D'])
df.plot(kind = 'bar', stacked=True); plt.show();
```

incorrectly produces this plot

![screen shot 2014-09-04 at 1 38 47 pm](https://cloud.githubusercontent.com/assets/8483583/4153575/71c69b9e-345a-11e4-9d2d-cc0b00f4ee8b.png)
Notice that at '2' on the x-axis, there should be a bar of size -10 for each of the 'B' and 'C' categories.

However, when I replace the NaN values with 0s by doing

``` python
df = pd.DataFrame([[10,20,5,40],[-5,5,20,30],[np.nan,-10,-10,20],[10,20,20,-40]], columns = ['A','B','C','D'])
df = df.fillna(0)
df.plot(kind = 'bar', stacked=True); plt.show();
```

then the plot displays correctly

![screen shot 2014-09-04 at 1 41 41 pm](https://cloud.githubusercontent.com/assets/8483583/4153668/1e31ba26-345b-11e4-88a9-e7a0a764365b.png)

This is clearly not a good behaviour. I suspect that this happens because the bars corresponding to the negative values are trying to use np.nan as their 'bottom' argument and thus not displaying at all, but I haven't investigated further. 

It would be nice if area-style plots like this would either automatically replace NaN values with 0 or throw an error about NaN values present in the dataframe causing problems for the plotting functions.
