You can mix C and Fortran in Simply Fortran. We do ship a C++ compiler, but it isn't full-featured.
Plotting can be difficult. We provide a very simply plotting interface, Aplot, for making 2D plots (bar, scatter, line), but we don't provide anything beyond that with Aplot. You can look at what's available in the SF Package Manager, but those packages are provided as a convenience and a separate subscription. We didn't develop any of these libraries; we're simply making them available.
You can create a multi-window interface using AppGraphics, GTK+, and possibly other GUI toolkits (DISLIN, possibly?). However, if you want to integrate 3D graphics, you'll probably want to stick with GTK+ with PLPlot support.
We don't have an example of 3D plotting, but we have simpler examples. You'll notice "Conway's Game of Life" listed in the Start panel in Simply Fortran when you first start the development environment. That can provide a simple example of how AppGraphics works. The GTK-Fortran project provides some examples that should work with our package in the SF Package Manager.
What you're proposing is possible, but it isn't necessarily simple to demonstrate. You'll need to use some sort of Fortran-accessible GUI and plotting library to get where you want to be.