No working GUI backend found for matplotlib

I am getting the above warning when I try to use PyPlot for plotting and no plots show up. I followed the instructions here (https://github.com/JuliaPy/PyPlot.jl/issues/219), but it still didn't fix the issue when I try to call the Julia script which tries to use PyPlot. In the Julia terminal, after following those instructions, the warning does not show up but I also cannot get the basic example to show up. I use Julia v0.6.4 on MacOS. PyPlot used to work for me several months ago, and just tried to use it again today. I've done at least one Pkg.update() since the last time PyPlot worked.

Start by reproducing the basic PyPlot example in the Julia terminal and from the reported Julia script on macOS with Julia v0.6.4. Review the setup instructions in PyPlot.jl issue #219 and compare the GUI backend behavior in both contexts. Done means the warning is resolved and the basic example displays a plot.

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