Chord diagram problem, question marks in boxes
I am using version 3.2.3+dfsg1-1~ppa1804+1 with Linux Mint 19.2 Cinnamon. Suddenly I've started getting a problem with chord diagrams: Above strings that should have no o or x mark, instead of a blank there's a question mark in a box. See attached PDF. If I try to change the mark it cycles between o, x, and question mark in box.