Add a distribution plot for total chi²
Created by: Zaharid
We would like to have plots similar to
that work not only for a single dataset, but for arbitrary aggregates of datasets, including in particular the total chi².
Bonus points if we can have kde plots as well (https://github.com/NNPDF/nnpdf/blob/d5f6751d6693f14eec7c40e7bc5589afb82d8e2d/validphys2/src/validphys/plotutils.py#L392).
Note that this will need to be modified to work for #651 .