Plots did not seem defined in the .toml and had therefore to be installed manually.

[jellepoland]

I fixed it by running:

using Pkg
Pkg.activate(".")
Pkg.add("Plots")

This gave new errors, which were resolved by updating a version of openssl

Which itself gave new errors, requiring a reinstallation of the package.

Afterwhich everything, including the plots, were working smoothly.

Not sure what caused the problem.

[1-Bart-1]

We use ControlPlots for plotting, and we don't want ControlPlots to be a direct dependency of the package, in case you don't need plotting or want to use a different plotting library. You can either install ControlPlots in your global environment:

julia -e 'using Pkg; Pkg.add("ControlPlots")'

Or, what we do in the examples, activate the TestEnv:

    using TestEnv; TestEnv.activate()

[jellepoland]

Okay, I see, thanks.
I am still unsure why I needed to go through some hoops before the code ran, but I will try reinstalling at some point and see if I find the same problems.
Let's close this for now :)