diff --git a/cameo/gnuplot.c b/cameo/gnuplot.c
index 566567b..d02e8e0 100644
--- a/cameo/gnuplot.c
+++ b/cameo/gnuplot.c
@@ -88,7 +88,8 @@ static int gnuplot_do_write(FILE *file, const struct path *paths)
 		if (path != paths)
 			if (fprintf(file, "\n") < 0)
 				return 0;
-		if (fprintf(file, "#%%r_tool=%f\n", path->r_tool) < 0)
+		if (path->r_tool &&
+		    fprintf(file, "#%%r_tool=%f\n", path->r_tool) < 0)
 			return 0;
 		if (path->outside && fprintf(file, "#%%outside\n") < 0)
 			return 0;