diff -Naur gnuplot-4.2.0-orig/src/util.c gnuplot-4.2.0-thousands_separator/src/util.c
--- gnuplot-4.2.0-orig/src/util.c	2007-04-02 13:39:48.000000000 +0200
+++ gnuplot-4.2.0-thousands_separator/src/util.c	2007-04-02 13:41:21.000000000 +0200
@@ -567,7 +567,8 @@
 	*t++ = '%';
 	/* dont put isdigit first since sideeffect in macro is bad */
 	while (*++format == '.' || isdigit((unsigned char) *format)
-	       || *format == '-' || *format == '+' || *format == ' ')
+         || *format == '-' || *format == '+' || *format == ' '
+         || *format == '\'')
 	    *t++ = *format;
 	/*}}} */