Use DataFrame.index for x-axis

It is typical to use a `DatetimeIndex` on Pandas `DataFrame` and it works well when plotting/indexing so it would be nice to be able to specify e.g.

```
cvs = ds.Canvas(x_range=x_range, y_range=y_range, plot_width=w, plot_height=h)
cvs.line(df, 'index', 'y', agg=ds.any())
```

to draw a line.

Furthermore, if using Dask `DataFrame` that loads out-of-memory data lazily, it is necessary to specify the range using the index or it will go through the entire `DataFrame` every time which can be very slow.