gfio: fix discrepancies between graph header and implementation

Header defined a couple functions as returning void, but they really return int.

Signed-off-by: Stephen M. Cameron <stephenmcameron@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/graph.c b/graph.c
index d7bdbe5..1194510 100644
--- a/graph.c
+++ b/graph.c
@@ -29,6 +29,7 @@
 #include <gtk/gtk.h>
 
 #include "tickmarks.h"
+#include "graph.h"
 
 struct xyvalue {
 	double x, y;
diff --git a/graph.h b/graph.h
index 39dd2b0..7c3e862 100644
--- a/graph.h
+++ b/graph.h
@@ -14,8 +14,8 @@
 void graph_x_title(struct graph *g, const char *title);
 void graph_y_title(struct graph *g, const char *title);
 void graph_add_label(struct graph *g, const char *label);
-void graph_add_data(struct graph *g, const char *label, const double value);
-void graph_add_xy_data(struct graph *g, const char *label,
+int graph_add_data(struct graph *g, const char *label, const double value);
+int graph_add_xy_data(struct graph *g, const char *label,
 		const double x, const double y);
 void graph_set_color(struct graph *g, const char *label,
 		double red, double green, double blue);