can't plot multi-row subplots
This code works fine to produce a single row of 3 subplots

``` python
g = np.random.choice([1,2,3], 10)
s = np.random.normal(size=10)
s2 =np.random.normal(size=10)
df = pd.DataFrame([g, s, s2]).T
df.columns = ['key', 's1', 's2']
gb = df.groupby('key')

fig, axes = plt.subplots(nrows=1, ncols=3, figsize=(12, 4))
i = 0
for key, df2 in gb:
    df2.plot(ax=axes[i], x='s1', y='s2', title=key)
    i = i + 1
```

But if I try to add a second row (nrows=2), it blows up with an Attribute Error

AttributeError: 'numpy.ndarray' object has no attribute 'get_figure'

``` python
fig, axes = plt.subplots(nrows=2, ncols=3)
fig.tight_layout() # Or equivalently,  "plt.tight_layout()"
i = 1
for key, df2 in gb:
    df2.plot(ax=axes[i])
    i = i + 1
```
