JuliaSim does have graphical representation, a GUI and an IDE. But, granted, it is also a commercial product, not open source.

But this representation is exclusive to JuliaSim, is it not? MTK package didn't have equivalents of Dialog, Icon and Diagram annotations the last time I checked.

It is exclusive to JuliaSim. But the tools generate MTK compatible component models so, once built, they can be used with MTK.

There' s a Chinese company also using Julia and Modelica https://www.tongyuan.cc/product/MWorks . They have projects like https://github.com/Suzhou-Tongyuan/syslab-deploy using https://discourse.julialang.org/t/syslabcc-suzhou-tongyuans-... .