Packages not being found on JULIA_DEPOT_PATH

I am getting

using Gadfly
ERROR: ArgumentError: Package Gadfly not found in current path:
- Run `import Pkg; Pkg.add("Gadfly")` to install the Gadfly package.

but I have

julia> ENV
ENV
Base.EnvDict with 29 entries:
  "JULIA_DEPOT_PATH"        => "/Users/dom/.julia:/nix/store/jmi07zhnpmnphnw3ls0cxmrq1n89spbd-julia-depot"

and

dom@MacBook-Pro depot % ls -ltr /nix/store/jmi07zhnpmnphnw3ls0cxmrq1n89spbd-julia-depot/compiled/v1.5/Gadfly/ 
<nw3ls0cxmrq1n89spbd-julia-depot/compiled/v1.5/Gadfly/ 
total 3296
-r-xr-xr-x  1 dom  admin  1686699  1 Jan  1970 DvECm_9igGY.ji

so what is stopping julia finding gadfly?

See my notes here: I am getting ```using Gadfly ERROR: ArgumentError: Package Gadfly not found in cu - #2 by idontgetoutmuch