@tobias.knopp: I think one of the most promising efforts was/is Winston.jl. It is pure Julia and thus is fully hackable. Integration with Gtk.jl works nicely.
Unfortunately the development behind this stalled a little bit.
I am not certain if this satisfies your needs, but the InspectDR plotting backend is also “pure Julia”: The rendering is done through Cairo.jl, and the interactive GUI makes use of Gtk.jl.
Link: ANN: New interactive Plots.jl backend for large datasets