Certain slices of Series can cause problems with matplotlib boxplot
We just encountered a problem when boxplotting a slice of a Series that does not contain the first element (i.e., the element with index 0). Here is an example:

``` python
plt.boxplot(pd.Series([1,2,3,4])[1:])
```

Which gives the following output:

```
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-29-b88183a29efb> in <module>()
----> 1 boxplot(pd.Series([1,2,3,4])[1:])

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/pyplot.pyc in boxplot(x, notch, sym, vert, whis, positions, widths, patch_artist, bootstrap, usermedians, conf_intervals, hold)
   2425                          positions=positions, widths=widths,
   2426                          patch_artist=patch_artist, bootstrap=bootstrap,
-> 2427                          usermedians=usermedians, conf_intervals=conf_intervals)
   2428         draw_if_interactive()
   2429     finally:

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/matplotlib/axes.pyc in boxplot(self, x, notch, sym, vert, whis, positions, widths, patch_artist, bootstrap, usermedians, conf_intervals)
   5744         if hasattr(x, 'shape'):
   5745             if len(x.shape) == 1:
-> 5746                 if hasattr(x[0], 'shape'):
   5747                     x = list(x)
   5748                 else:

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/series.pyc in __getitem__(self, key)
    468     def __getitem__(self, key):
    469         try:
--> 470             return self.index.get_value(self, key)
    471         except InvalidIndexError:
    472             pass

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/index.pyc in get_value(self, series, key)
    676         """
    677         try:
--> 678             return self._engine.get_value(series, key)
    679         except KeyError, e1:
    680             if len(self) > 0 and self.inferred_type == 'integer':

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/lib.so in pandas.lib.IndexEngine.get_value (pandas/src/tseries.c:123878)()

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/lib.so in pandas.lib.IndexEngine.get_value (pandas/src/tseries.c:123693)()

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/lib.so in pandas.lib.IndexEngine.get_loc (pandas/src/tseries.c:124430)()

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/lib.so in pandas.lib.Int64HashTable.get_item (pandas/src/tseries.c:18340)()

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/lib.so in pandas.lib.Int64HashTable.get_item (pandas/src/tseries.c:18284)()

KeyError: 0
```
