--- loncom/homework/functionplotresponse.pm	2010/10/09 17:43:25	1.10
+++ loncom/homework/functionplotresponse.pm	2010/10/09 20:02:02	1.11
@@ -1,7 +1,7 @@
 # LearningOnline Network with CAPA
 # option list style responses
 #
-# $Id: functionplotresponse.pm,v 1.10 2010/10/09 17:43:25 www Exp $
+# $Id: functionplotresponse.pm,v 1.11 2010/10/09 20:02:02 www Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -178,10 +178,10 @@ sub generate_input_field {
 #
 sub new_coordinate {
     my ($id,$variable,$x,$y)=@_;
-    if ($Apache::functionplotresponse::previous{&field_name($id,$variable,'x')}) {
+    if (defined($Apache::functionplotresponse::previous{&field_name($id,$variable,'x')})) {
        $x=$Apache::functionplotresponse::previous{&field_name($id,$variable,'x')};
     }
-    if ($Apache::functionplotresponse::previous{&field_name($id,$variable,'y')}) {
+    if (defined($Apache::functionplotresponse::previous{&field_name($id,$variable,'y')})) {
        $y=$Apache::functionplotresponse::previous{&field_name($id,$variable,'y')};
     }
     &generate_input_field($id,$variable,$x,$y);