I know a few tricks for passing arrows under other arrows in xymatrix such as:
\ar'[r][rr]
or
\ar[r]|{\hole}
But in a large diagram (many rows and columns) that has been shrunk down to a reasonable size, these spaces are all but invisible. (Though they can still be seen with a magnifying glass.) Is there any way to make the holes bigger so that they are visible to the naked eye?