>But I would love to be able to "shift" everything so that zero always refers
>to the middle vertical line (x) and the middle horizontal line (y) when
>writing code, I mean:

You can draw everything around the origin, them move all items by adding an x and y displacement
using the canvas move command.

>
>Instead of writing
>
>$::w.outer.canv create line 0 540 1920 540 -fill black
>
>I would write
>
>$::w.outer.canv create line -540 0 540 0 -fill black
>
>So the leftmost edge is now negative and the center is zero.
>
>I read the manual page and found a section that explains about COORDINATES
>and COORDINATES, but I don't really understand what it says. For example,
>
>"Canvases do not support scaling or rotation of the canvas coordinate system
>relative to the window coordinate system."

However they do support scrolling. See the -scrollregion configuration option and the canvas xview and yview commands. If you set -scrollregion size to match the window size, the whole scroll region will be displayed (xview and yview are not needed).

Dave B