Hi, I am trying to make scatter plot for large data with `holoviews.operation.datashader.rasterize`

and show the hover. But I find one dimension of the data is missed on the hover after I use the `rasterize`

. Here is the minimal example code:

```
import numpy as np
import datashader
import geoviews as gv
gv.extension('bokeh')
from holoviews.operation.datashader import rasterize
lon = np.array([8.75,8.76,8.77])
lat = np.array([46.1,46.2,46.3])
height = np.array([0.0,0.1,0.2])
index = np.array([0,1,2])
points = gv.Points(np.stack((lon,lat,height,index),axis=-1),
kdims=['Longitude','Latitude'],vdims=['Height','Index'])
points.opts(tools=['hover'])
```

The result looks perfect, `Longitude`

, `Latitude`

, `Height`

and `Index`

are all showed. (sorry I can only upload one image as a new user on this website. So, I upload the most important one.)

The I rasterize it:

```
points = rasterize(points,aggregator=datashader.mean('Height'),vdim_prefix='',precompute=True)
points.opts(tools=['hover'])
```

The `Index`

dimension is missing on the hover.

Then I try to print the rasterized `points`

:

```
print(points)
```

and get:

```
:DynamicMap []
:Image [Longitude,Latitude] (Height)
```

It looks like the fourth dimension is missed. I have searched the overall manual but didn’t find anything related to it. Please let me know if anybody knows how to preserve this `Index`

dimension and show it on the hover. Thanks a lot!