What simple graphics program can do this?

I’ll try my best to explain

http://oi57.tinypic.com/2yyyvzb.jpg
So this image may help.

I want a program where I can draw cubes and have a transparent section connecting one cube to another cube.

But I also want it so I can easily move the cubes and still show the transparent section between the two cubes.

I want it so when multiple transparent sections overlap that it will be brighter or even a different colour.