My defaultfigurerenderer was NOT 'opengl' and thus the transparency didn't render. There was no option within the app to fix this. I closed the app, and in the command line, did
and then reran the app and it worked. Consider setting the figure's renderer manually to opengl when you create it so you don't have to rely on user's potentially different default settings.
Another suggestion, have the sliders update the slices dynamically as you slide them. This can be done and there's material online to do this. I think I've used addListener to check for changes in the slider value, which will be changing as it slides.
Great functionality so far and a nice interface. Great job!