Sufficiently developed? No way. Is it getting there? Yes I think. There are no plotting facilities part of the standard library, but external packages like Winston [1] are available -- but the syntax was verbose for my taste, so I let it pass.

According to this SO question[2], there was some web-REPL plotting earlier but it is unmaintained now.

[1] https://github.com/nolta/Winston.jl

[2] http://stackoverflow.com/questions/14970817/plot-not-defined...