Text alignment problems

[peter boardman]

I'm importing an OmniOutline document into OmniGraffle. 

My problem is that the ascenders of letters in text objects are
touching the tops of the boxes, when set to ArialMT font.

Generally, the letters move up and down inside the boxes depending
on what font I select for them. Some fonts align correctly: Helvetica,
for example - but for other fonts the letters are too high up inside the
box. It's not just ArialMT: also AntiqueOlive, ITC Bauhaus, Friz
Quadrata, News Gothic, ITC Avant Garde Gothic, Tekton, ITC
Officina Serif to name a few.

Why is it doing this? What can I do, apart from using a specific font,
or exporting via PDF to Illustrator?

Pete