diff --git a/ubb-la/gui.c b/ubb-la/gui.c
index d83fecc..6702f88 100644
--- a/ubb-la/gui.c
+++ b/ubb-la/gui.c
@@ -40,6 +40,7 @@
 #define	UNIT_RGBA	0xb0e0ffff	/* units */
 #define	MAP_BUF_RGBA	0x808080ff	/* buffer in the map */
 #define	MAP_VIEW_RGBA	0xffffffff	/* current view in the map */
+#define	CENTER_RGBA	0x5080ffff	/* center marker */
 #define	LEVEL_RGBA	0xffff00ff	/* constant level or single change */
 #define	BOUNCE_RGBA	0xff8080ff	/* bouncing signal */
 #define	LABEL_RGBA	0xffffffff	/* channel label */
@@ -57,6 +58,10 @@
 #define	MAP_VIEW_Y0	0
 #define	MAP_VIEW_Y1	10
 
+#define	CENTER_W	8
+#define	CENTER_Y0	20
+#define	CENTER_Y1	24
+
 #define	CH_XOFF		30
 #define	CH_YOFF		30
 #define	CH_SKIP		16
@@ -381,7 +386,8 @@ static void show_divisions(void)
 
 	for (i = -n; i <= n; i++)
 		vlineColor(surf, XCENTER+i*DIV_X,
-		    ch_y(0, 1)-DIV_Y, ch_y(3, 0)+DIV_Y, DIV_RGBA);
+		    ch_y(0, 1)-DIV_Y, ch_y(3, 0)+DIV_Y,
+		    i ? DIV_RGBA : CENTER_RGBA);
 }
 
 
@@ -410,6 +416,9 @@ void gui(const uint8_t *buf, int skip, int nibbles, double freq)
 		for (i = 0; i != 4; i++)
 			textf(0, ch_y(i, 1), LABEL_RGBA, "CH%d", i);
 		show_divisions();
+		filledTrigonColor(surf, XCENTER, CENTER_Y1,
+		    XCENTER-CENTER_W/2, CENTER_Y0,
+		    XCENTER+CENTER_W/2, CENTER_Y0, CENTER_RGBA);
 		show_buffer(buf, skip, nibbles, CH_XOFF, XRES, zoom, pos);
 		show_freq(freq, zoom);
 		update();