OpenBSD also needs the _XOPEN_SOURCE_EXTENDED define.

Index: ttyplot.c
--- ttyplot.c.orig
+++ ttyplot.c
@@ -8,7 +8,7 @@
 
 // This is needed on macOS to get the ncurses widechar API, and pkg-config fails to
 // define it.
-#ifdef __APPLE__
+#if defined (__APPLE__) || defined (__OpenBSD__)
 #define _XOPEN_SOURCE_EXTENDED
 #else
 // This is needed for musl libc