Thank you for bringing this up. I'm working with someone else on porting the code to Linux and they have also mentioned this as a side issue because of their default font selection on their computer.
Version 1.1 of Plans Unfolding has incorporated a fix for this problem (it specifies a default font and also allows the user to change it). It is downloadable from here
Thanks again for bringing this to my attention. This is the sort of thing that is hard to trace without the help of users on various computers.