Having legend sorted by bucket

[rcazorla]

Right now, when charts use multiple bucket, in case you have data of the same type in the buckets, it's difficult to see at first glance which data belong to which bucket in the legend :

As the field are sorted by bucket, having the name of the field could provide a better understanding to user. As such the example above would become :

[bertol83]

+1

[trevan]

I believe this is a duplicate of #3402

[spalger]

@trevan yes it is, thanks