GUI: Render "Loading" or similar before trying to render plot
Plots can take a long time to render, and until they do you get a blank window. At least on Linux, the window is full of whatever happened to be in the background when the window was created. It looks buggy and doesn't give any clues as to what the window is.
Proposed: Before doing the real plot, render a fake plot that just contains the text "Loading...", "Rendering plot..." or similar.
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