TypeError: invalid type promotion
I'm not sure where to post this, as it involves pandas, seaborn, matplotlib and numpy.

As it works with pandas 0.20.3, but throws an error in pandas 0.21.0, it seems to be a pandas issue. Below, an easily reproducible example:

```python
import pandas as pd
import seaborn as sns

index = pd.date_range('2017-06-01', periods=16)
df = pd.DataFrame(index=index, columns=['a', 'b', 'c'])
df.iloc[:2, 0] = '1000'
df.iloc[[2, 3, 6, 8, 10, 12, 14, 15], 1] = '1000'
df.iloc[[4, 5, 9, 13], 2] = '1000'
df.iloc[[7, 11], 1] = '1001'
df.index.name = 'Datetime'
df.columns.name = 'Element'
df = df.stack().rename('Event').reset_index()
df['Event / Element'] = df['Event'] + ' / ' + df['Element']
sns.stripplot(x='Datetime', y='Event / Element', hue='Element', data=df, orient='h')
```
Throws following error in pandas 0.21.0:
```
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\IPython\core\interactiveshell.py", line 2881, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-2-a21ce0484562>", line 14, in <module>
    sns.stripplot(x='Datetime', y='Event / Element', hue='Element', data=df, orient='h')
  File "C:\Python27\lib\site-packages\seaborn\categorical.py", line 2603, in stripplot
    plotter.plot(ax, kwargs)
  File "C:\Python27\lib\site-packages\seaborn\categorical.py", line 1191, in plot
    self.draw_stripplot(ax, kws)
  File "C:\Python27\lib\site-packages\seaborn\categorical.py", line 1171, in draw_stripplot
    ax.scatter(strip_data, cat_pos, **kws)
  File "C:\Python27\lib\site-packages\matplotlib\__init__.py", line 1710, in inner
    return func(ax, *args, **kwargs)
  File "C:\Python27\lib\site-packages\matplotlib\axes\_axes.py", line 4087, in scatter
    offsets = np.column_stack([x, y])
  File "C:\Python27\lib\site-packages\numpy\lib\shape_base.py", line 353, in column_stack
    return _nx.concatenate(arrays, 1)
TypeError: invalid type promotion
```
numpy==1.13.3
seaborn==0.8.1
matplotlib==2.1.0

The above code does work fine in pandas 0.20.3, with same versions of numpy, seaborn and matplotlib.