diff --git a/graph_gui.c b/graph_gui.c
--- a/graph_gui.c
+++ b/graph_gui.c
@@ -1,6 +1,7 @@
 #include "stdafx.h"
 #include "openttd.h"
 #include "table/strings.h"
+#include "functions.h"
 #include "window.h"
 #include "gui.h"
 #include "gfx.h"