Normalized coordinates for plots/graphics?

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Is it possible to use normalized (unit) coordinates for graphics?

I would like to position a text label below the x-axis of a plot. Something like this:

po = plot(...)
po += text( (r'$mylabel$', (.5, -.1))

Where (.5, -.1) are coordinates relative to the bounding box of the plot with the extends (0..1,0..1). So yes, this is a very simple coordinate transformation. Do I have to do it by hand? And if so, how do I get the actual bounding box of the plot?