At some level some library will call out to some code that isn’t Julia. It’s just a question of how deeply buried you need it to be. With PyPlot, Python is probably too close to the surface for you, is that correct? Even though all your code will be in Julia? While Gadfly calling out to Compose calling a C-library for drawing primitives is ok?
I would think that Gadfly.jl or Makie.jl should be ok. Plots.jl are considered native Julia, but check out the various backends to see how early they call out to other languages.
But please note, that by your standards, matplotlib probably isn’t native Python, so “Doing math with python”, should rather be “Doing math with python and c++”.