diff --git a/graph b/graph
index 311e4e0..20da785 100755
--- a/graph
+++ b/graph
@@ -247,7 +247,10 @@ else
 	now=$(date +%s)
 	cat <<-EOHEAD
 		content-type: image/png
-		Last-Modified: $(LC_ALL=C TZ=GMT date +'%a, %d %b %Y %X %Z')
+		Last-Modified: $(
+			LC_ALL=C TZ=GMT date +'%a, %d %b %Y %X %Z'	\
+			-d @$(rrdtool last ingress.rrd)
+		)
 		Expires: $(LC_ALL=C TZ=GMT date +'%a, %d %b %Y %X %Z' -d '1 hour')
 		Cache-Control: max-age=0, must-revalidate