As far as I understand (this thread has a long discussion about it), all the julia plotting packages apart from UnicodePlots.jl call deep down a library in another language. Gadfly calls Cairo, Plots with the default backend calls GR which is written in C, and each of Makie’s backends calls their own library, Cairo for Cairo Makie, OpenGL for GLMakie and WebGL for WGLMakie. So in that sense even Gadfly is not written in pure julia and the “drawing” functions are in another library.
If that is acceptable for you maybe Cairo.jl has some of the most transparent examples: Cairo.jl/Samples.md at master · JuliaGraphics/Cairo.jl · GitHub