Ah I understand. I was actually looking for the latter (background color outside the canvas).
With some trial & error I got this, which looks quite close to what I was looking for. Here’s the additional code, in case it helps someone else:
bx0 = 360.;
by0 = -0.6;
bw = 350;
bh = 0.6;
plot!(plt3[1], Shape(bx0 .+[0,bw,bw,0], by0.+[0,0,bh,bh]), fc=:white,
lc=:green, lw=2, opacity=.85, label=nothing);
Positioning the box was messy so it would be great if Julia supports something like this natively!
