diff -ru grace-5.1.23.orig/src/ssdata.c grace-5.1.23/src/ssdata.c
--- grace-5.1.23.orig/src/ssdata.c	2015-03-01 16:13:28.394687611 -0500
+++ grace-5.1.23/src/ssdata.c	2015-03-01 16:06:15.303700533 -0500
@@ -275,8 +275,14 @@
             (*nncols)++;
         } else {
             /* last resort - treat the field as string, even if not quoted */
-            (*formats)[ncols] = FFORMAT_STRING;
-            (*nscols)++;
+	    if (!strncmp(token, "inf", 3) || !strncmp(token, "-inf", 4) ||
+	        !strncmp(token, "nan", 3) || !strncmp(token, "-nan", 4)) {
+		(*formats)[ncols] = FFORMAT_NUMBER;
+		(*nncols)++;
+	    } else {
+                (*formats)[ncols] = FFORMAT_STRING;
+                (*nscols)++;
+	    }
         }
     }
     xfree(buf);