Are data.group or data.join methods available ?

```
Hi I would like to group the data in the Java client part through the use of 
the javascript visualization.data.group method.
I didn't find the mapping in the library. Does the JSNI mapping exist ?
thank for your help
```

Original issue reported on code.google.com by `ooooh.g...@gmail.com` on 17 Mar 2015 at 2:36
