dfr mean for expression> dfr mean {flags} 
| input | output | 
|---|---|
| any | any | 
Mean value from columns in a dataframe
> [[ab]; [62] [42] [22]] |dfr into-df|dfr mean
╭───┬──────┬──────╮
│# │  a   │  b   │
├───┼──────┼──────┤
│0│4.00│2.00│
╰───┴──────┴──────╯
Mean aggregation for a group-by
> [[ab]; [one2] [one4] [two1]]
|dfr into-df
|dfr group-bya
|dfr agg (dfr colb|dfr mean)
╭───┬─────┬──────╮
│# │  a  │  b   │
├───┼─────┼──────┤
│0│one│3.00│
│1│two│1.00│
╰───┴─────┴──────╯
Tips: Dataframe commands were not shipped in the official binaries by default, you have to build it with --features=dataframe flag
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    https://www.nushell.sh/commands/docs/dfr_mean.html