This is packaging the vega command line tools for nix (https://vega.github.io/vega/usage/#cli). Vega is a handy declarative visualization tool and this cli allows to convert vega specifications directly to svg/pdf/png instead of looking at them online.
Basically what I did was to run:
nix-shell -p nodePackages.node2nix --command "node2nix -i <(echo '[\"vega-cli\", \"vega-lite\"]')"
Then I added a few extra dependencies to the generated default.nix
such that the last lines look like this: