Checkbox with dash?

[Paul Foraker]

The Mac interface guidelines indicate that a checkbox has two states,
checked and unchecked. Except when it has a third state, dashed, indicating
that not all the conditions are met for checking or unchecking.

I have a field "states" containing a list of the states of the U.S. and a
set of checkboxes for the regions: Northeast, East, West, etc.  If the user
clicks Northeast, the checkbox gets checked, and the associated states get
hilited in the field. If the user then clicks in the "states" field to
unhilite New York, the state of the Northeast checkbox should change to
"dashed".

I tried converting the checkboxes to a list in a field and setting char 1
of each line to an image of a checkbox checked, unchecked, or dashed.  That
doesn't work because the baseline of the name of the region appears at the
baseline of the image; that is, too low. The label (Northeast) needs to
appear at the midline of the image. I don't see a way to move the image
down on the line.

Another approach would be to put a transparent field containing a dash over
the checkbox, in its unchecked state. Handling the location and multiple
ways the state can change started to get awkward and unwieldy.

Is there a better way to do this?

Thanks
-- Paul