The docs say that the default font styling on the Xaxis labels is :'10px bold "Lucida Grande", "Lucida Sans Unicode",Verdana, Arial, Helvetica, sans-serif'
I'm pretty sure this isn't correct as when I actually set that exact value, I get different results than the default. It's not so obvious (but still different) in safari, but glaringly obvious in ie8. See attached photos.
Oh, apparently I can only attach 3 photos, but the ie8 ones are the ones that really show this anyway