diff --git a/overlay/chart.c b/overlay/chart.c
index 2a33d612ff251f6106da6934df10928b34b9af3e..734e911f2f78f8c04951544803710b4f3e775880 100644
--- a/overlay/chart.c
+++ b/overlay/chart.c
@@ -143,7 +143,11 @@ static double value_at(struct chart *chart, int n)
 	else if (n >= chart->current_sample)
 		n = chart->current_sample - 1;
 
-	return chart->samples[n % chart->num_samples];
+	n %= chart->num_samples;
+	if (n < 0)
+		n += chart->num_samples;
+
+	return chart->samples[n];
 }
 
 static double gradient_at(struct chart *chart, int n)