diff --git a/main.c b/main.c
index 23ccda4..37f679d 100644
--- a/main.c
+++ b/main.c
@@ -1442,7 +1442,7 @@ const struct {
 } trace_info[] = {
   { "LOGMAG", 7, 10 },
   { "PHASE",  4, 90 },
-  { "DELAY",  4,  1 },
+  { "DELAY",  4,  1e-9 },
   { "SMITH",  0,  1 },
   { "POLAR",  0,  1 },
   { "LINEAR", 0,  0.125 },
diff --git a/plot.c b/plot.c
index 234ca94..dff5354 100644
--- a/plot.c
+++ b/plot.c
@@ -439,6 +439,24 @@ float phase(float *v)
   return 2 * atan2f(v[1], v[0]) / M_PI * 90;
 }
 
+/*
+ * calculate groupdelay
+ */
+float groupdelay(float *v, float deltaf)
+{
+  float *w = &v[2]; // point to next coeff
+#if 1
+  // w = w[0]/w[1]
+  // v = v[0]/v[1]
+  // atan(w)-atan(v) = atan((w-v)/(1+wv))
+  float r = w[0]*v[1] - w[1]*v[0];
+  float i = w[0]*v[0] + w[1]*v[1];
+  return atan2f(r, i) / (2 * M_PI * deltaf);
+#else
+  return (atan2f(w[0], w[1]) - atan2f(v[0], v[1])) / (2 * M_PI * deltaf);
+#endif
+}
+
 /*
  * calculate abs(gamma)
  */
@@ -502,6 +520,12 @@ trace_into_index(int x, int t, int i, float coeff[2])
   case TRC_PHASE:
     v = refpos - phase(coeff) * scale;
     break;
+  case TRC_DELAY:
+    if (i != 100) {
+      float deltaf = frequencies[i+1] - frequencies[i];
+      v = refpos - groupdelay(coeff, deltaf) * scale;
+    }
+    break;
   case TRC_LINEAR:
     v = refpos + linear(coeff) * scale;
     break;
@@ -631,7 +655,7 @@ gamma2reactance(char *buf, int len, const float coeff[2])
 }
 
 static void
-trace_get_value_string(int t, char *buf, int len, float coeff[2], uint32_t frequency)
+trace_get_value_string(int t, char *buf, int len, float coeff[2], int i)
 {
   float v;
   switch (trace[t].type) {
@@ -646,6 +670,13 @@ trace_get_value_string(int t, char *buf, int len, float coeff[2], uint32_t frequ
     v = phase(coeff);
     chsnprintf(buf, len, "%.2f" S_DEGREE, v);
     break;
+  case TRC_DELAY:
+    {
+      float deltaf = frequencies[i+1] - frequencies[i];
+      v = groupdelay(coeff, deltaf);
+      string_value_with_prefix(buf, len, v, 's');
+    }
+    break;
   case TRC_LINEAR:
     v = linear(coeff);
     chsnprintf(buf, len, "%.2f", v);
@@ -655,7 +686,7 @@ trace_get_value_string(int t, char *buf, int len, float coeff[2], uint32_t frequ
     chsnprintf(buf, len, "%.2f", v);
     break;
   case TRC_SMITH:
-    gamma2imp(buf, len, coeff, frequency);
+    gamma2imp(buf, len, coeff, frequencies[i]);
     break;
   case TRC_REAL:
     chsnprintf(buf, len, "%.2f", coeff[0]);
@@ -1379,7 +1410,7 @@ cell_draw_marker_info(int m, int n, int w, int h)
     trace_get_info(t, buf, sizeof buf);
     cell_drawstring_5x7(w, h, buf, xpos, ypos, config.trace_color[t]);
     xpos += 64;
-    trace_get_value_string(t, buf, sizeof buf, measured[trace[t].channel][idx], frequencies[idx]);
+    trace_get_value_string(t, buf, sizeof buf, measured[trace[t].channel][idx], idx);
     cell_drawstring_5x7(w, h, buf, xpos, ypos, config.trace_color[t]);
     j++;
   }    
diff --git a/ui.c b/ui.c
index 3cbac63..0344c30 100644
--- a/ui.c
+++ b/ui.c
@@ -68,7 +68,7 @@ enum {
 };
 
 enum {
-  KM_START, KM_STOP, KM_CENTER, KM_SPAN, KM_CW, KM_SCALE, KM_REFPOS, KM_EDELAY, KM_VELOCITY_FACTOR
+  KM_START, KM_STOP, KM_CENTER, KM_SPAN, KM_CW, KM_SCALE, KM_REFPOS, KM_EDELAY, KM_VELOCITY_FACTOR, KM_SCALEDELAY
 };
 
 uint8_t ui_mode = UI_NORMAL;
@@ -742,12 +742,16 @@ static void
 menu_scale_cb(int item)
 {
   int status;
+  int km = KM_SCALE + item;
+  if (km == KM_SCALE && trace[uistat.current_trace].type == TRC_DELAY) {
+    km = KM_SCALEDELAY;
+  }
   status = btn_wait_release();
   if (status & EVT_BUTTON_DOWN_LONG) {
-    ui_mode_numeric(KM_SCALE + item);
+    ui_mode_numeric(km);
     ui_process_numeric();
   } else {
-    ui_mode_keypad(KM_SCALE + item);
+    ui_mode_keypad(km);
     ui_process_keypad();
   }
 }
@@ -1221,13 +1225,14 @@ const keypads_t * const keypads_mode_tbl[] = {
   keypads_freq, // span
   keypads_freq, // cw freq
   keypads_scale, // scale
-  keypads_scale, // respos
+  keypads_scale, // refpos
   keypads_time, // electrical delay
-  keypads_scale // velocity factor
+  keypads_scale, // velocity factor
+  keypads_time // scale of delay
 };
 
 const char * const keypad_mode_label[] = {
-  "START", "STOP", "CENTER", "SPAN", "CW FREQ", "SCALE", "REFPOS", "EDELAY", "VELOCITY%"
+  "START", "STOP", "CENTER", "SPAN", "CW FREQ", "SCALE", "REFPOS", "EDELAY", "VELOCITY%", "DELAY"
 };
 
 void
@@ -1487,6 +1492,9 @@ fetch_numeric_target(void)
   case KM_VELOCITY_FACTOR:
     uistat.value = velocity_factor;
     break;
+  case KM_SCALEDELAY:
+    uistat.value = get_trace_scale(uistat.current_trace) * 1e12;
+    break;
   }
   
   {
@@ -1706,6 +1714,9 @@ keypad_click(int key)
     case KM_VELOCITY_FACTOR:
       velocity_factor = value;
       break;
+    case KM_SCALEDELAY:
+      set_trace_scale(uistat.current_trace, value * 1e-12); // pico second
+      break;
     }
 
     return KP_DONE;