Plot of streamz plots all data so far

In the example code, the resultant stream only ever contains two entries, but the hvplot output only adds data, and grows forever

```
import streamz
s = streamz.dataframe.Random(freq='1s', interval='3s')

tail = s.tail(2) # tail always has two elements
table = tail.hvplot(kind='table')
```

This does not happen if the plot occurs on a window object as opposed to a raw dataframe.


Also notice that the index has gone (it shows correctly as the x-axis in a line plot)