For a diagram, "isometric" (no vanishing point) perspective might be sufficient. You can simulate that easily with a horizontal skew.
Some time ago, I was playing with computing tranformations simulating the changes of viewpoint, both in scripts for animated svg
http://www.volny.cz/pavel.adamek/svg/cube/cube.htm
http://www.volny.cz/pavel.adamek/svg/rubik/rubik.htm
(Tested in Internet Explorer and Opera. For enabling of scripts contained in a svg document when viewing in Internet Explorer (6.0), the "active scripting" must be set to "Enable"; seting it to "Ask" is not enough.)
and as an effect for Inkscape
http://www.volny.cz/pavel.adamek/svg/house/house.htm
I had not enough time to develop it to something usable, but maybe this can serve as an inspiration for somebody with more skill.
P.A.