There are none built-in to flot.
Your best bet is to do it yourself via positioned divs, but if you are adventurous, you can look at the issue (Or the original issue) and see how other people have dealt with it.
Specifically, there are two people who have recently made label-related revisions to flot:
https://github.com/RuiPereira/flot/raw/axislabels/jquery.flot.axislabels.js