Submodule needs pyplot and it breaks documenter

I am building a package and have a submodule that uses PyPlot. The main package does not. When I try

using MyPkg

I get a complaint unless PyPlot is in the Project.toml. Having put it in and pushed the new material, I get this from documenter (using github actions)

1m 10s
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) <class 'ModuleNotFoundError'>
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Run julia --project=docs --color=yes docs/make.jl
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ERROR: LoadError: InitError: PyError (PyImport_ImportModule
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The Python package matplotlib could not be found by pyimport. Usually this means
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that you did not install matplotlib in the Python version being used by PyCall.
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PyCall is currently configured to use the Python version at:
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/usr/bin/python3
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and you should use whatever mechanism you usually use (apt-get, pip, conda,
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etcetera) to install the Python package containing the matplotlib module.
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One alternative is to re-configure PyCall to use a different Python
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version on your system: set ENV["PYTHON"] to the path/name of the python
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executable you want to use, run Pkg.build("PyCall"), and re-launch Julia.
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Another alternative is to configure PyCall to use a Julia-specific Python
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distribution via the Conda.jl package (which installs a private Anaconda
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Python distribution), which has the advantage that packages can be installed
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and kept up-to-date via Julia.  As explained in the PyCall documentation,
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set ENV["PYTHON"]="", run Pkg.build("PyCall"), and re-launch Julia. Then,
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To install the matplotlib module, you can use `pyimport_conda("matplotlib", PKG)`,
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where PKG is the Anaconda package the contains the module matplotlib,
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or alternatively you can use the Conda package directly (via
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`using Conda` followed by `Conda.add` etcetera).
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) <class 'ModuleNotFoundError'>

Is there anything I can do about this?