Plotting of streamz apply is odd

In the following code
```
data = a streamz dataframe with time-based index and example
recent = data.window(value='5m')
ts = recent.apply(lambda d: d.time.resample('10s').count())
```

- `ts.hvplot(kind='table')` works (but time index doesn't show, #199 )
- `ts.hvplot(kind='bar')` fails with
```
~/anaconda/envs/py36/lib/python3.6/site-packages/pandas/core/internals/managers.py in insert(self, loc, item, value, allow_duplicates)
1147 if not allow_duplicates and item in self.items:
1148 # Should this be a different kind of error??
-> 1149 raise ValueError('cannot insert {}, already exists'.format(item))
1150
1151 if not isinstance(loc, int):

ValueError: cannot insert time, already exists
```
- `ts.hvplot()` does not error but does not work
```
bokeh backend could not plot any Elements in the Overlay.
:DynamicMap []
:NdOverlay [Variable]
```