diff --git a/src/include/ngspice/graph.h b/src/include/ngspice/graph.h
index 02cdb0e0d..b69634f17 100644
--- a/src/include/ngspice/graph.h
+++ b/src/include/ngspice/graph.h
@@ -17,7 +17,7 @@ Author: 1988 Jeffrey M. Hsu
 
 struct _keyed;
 
-#define GRAPH_UNITS_LENGTH  20
+#define GRAPH_UNITS_LENGTH  21
 
 /* Device-independent data structure for plots. */