This is a first draft of a "digital graph paper program". It depends
on pygoocanvas and therefore goocanvas, which has just hit Debian
unstable, but isn't guaranteed to be available on your distro of
choice.

Lines are drawn every 20 units (adjustable by SPACE variable, but
probably using zoom would be better if you wanted to change
that). Since pixels are located at 0.5 units offset from the main
grid, we have to decide where to draw the graph paper lines. If you
choose to draw them at -0.5 (and therefore 19.5), which makes each
line bind to the square to its left (or top). If you choose to draw
them at +0.5 (and therefore 20.5), then each line binds to the square
on its right (or bottom). (In this context, "bind" means "is drawn in
the area of".) For this application it doesn't matter much, since
clicking on or near lines is intended to select the line, not a
square.