This fixes an issue where grouped data requests in the web interface could be sent in the wrong format. The change helps prevent errors when users view or analyze grouped information, improving reliability without changing visible features.

A mistake introduced in 234db70d861a7bc9c6620f9ef6a24dbe98eabb98, the
`groupby` of  `_read_group` should be list/tuple of  `str`, not a `str`.