Take a look at DataVoyager.jl. It is not really a WYSIWYG plot editor, more a data exploration tool. But you can still create plots interactively, and then easily get the plot specification into your running julia session as a VegaLite.jl plot.
Once the vega team has created a new version of lyra that works with the current generation of vega, I plan to provide a similar tool to DataVoyager.jl, but for lyra. Lyra is much more of a WYSIWYG plot editor than voyager. There are some nice videos and an online version of the previous version here.