Sure, and I'm open to discussion about the cutoff. I noticed when I first played with this in the GUI my first two columns were Accession Numbers (unique to each line) and Patient ID (about 1000 IDs) ... the autoselect nature of the widget put those on the X/Y axis. The GUI spun for a few minutes before displaying everything.

I figured most of my categories were either single-digits (Gender, Race, drug status, etc.) or thousands (accession numbers, IDs, etc). Ten seemed a reasonable number.