diff --git a/.gitignore b/.gitignore
index 42c4b55..d2fc5aa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,4 +8,5 @@ python
 *.py
 *.ipynb
 TAGS
-.emacs-dirvars
\ No newline at end of file
+.emacs-dirvars
+*png
diff --git a/.vscode/tasks.json b/.vscode/tasks.json
new file mode 100644
index 0000000..dc6b6f8
--- /dev/null
+++ b/.vscode/tasks.json
@@ -0,0 +1,20 @@
+{
+    "version": "2.0.0",
+    "tasks": [
+        {
+            "label": "make",
+            "type": "shell",
+            "command": "make",
+            "group": "build"
+        },
+        {
+            "label": "flash",
+            "type": "shell",
+            "command": "make dfu flash",
+            "group": {
+                "kind": "build",
+                "isDefault": true
+            }
+        }
+    ]
+}
\ No newline at end of file
diff --git a/plot.c b/plot.c
index b79db58..b004fc4 100644
--- a/plot.c
+++ b/plot.c
@@ -442,12 +442,9 @@ float phase(float *v)
 /*
  * calculate groupdelay
  */
-float groupdelay(float *v, float deltaf)
+float groupdelay(float *w, 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];
@@ -505,12 +502,27 @@ cartesian_scale(float re, float im, int *xp, int *yp, float scale)
   *yp = HEIGHT/2 - y;
 }
 
+static float
+groupdelay_from_array(int i, float array[101][2])
+{
+  if (i == 0) {
+    float deltaf = frequencies[1] - frequencies[0];
+    return groupdelay(array[0], array[1], deltaf);
+  } else if (i == 100) {
+    float deltaf = frequencies[i] - frequencies[i-1];
+    return groupdelay(array[i-1], array[i], deltaf);
+  } else {
+    float deltaf = frequencies[i+1] - frequencies[i-1];
+    return groupdelay(array[i-1], array[i+1], deltaf);
+  }
+}
 
 uint32_t
-trace_into_index(int x, int t, int i, float coeff[2])
+trace_into_index(int x, int t, int i, float array[101][2])
 {
   int y = 0;
   float v = 0;
+  float *coeff = array[i];
   float refpos = 8 - get_trace_refpos(t);
   float scale = 1 / get_trace_scale(t);
   switch (trace[t].type) {
@@ -521,10 +533,7 @@ trace_into_index(int x, int t, int i, float coeff[2])
     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;
-    }
+    v = refpos - groupdelay_from_array(i, array) * scale;
     break;
   case TRC_LINEAR:
     v = refpos + linear(coeff) * scale;
@@ -655,8 +664,9 @@ gamma2reactance(char *buf, int len, const float coeff[2])
 }
 
 static void
-trace_get_value_string(int t, char *buf, int len, float coeff[2], int i)
+trace_get_value_string(int t, char *buf, int len, float array[101][2], int i)
 {
+  float *coeff = array[i];
   float v;
   switch (trace[t].type) {
   case TRC_LOGMAG:
@@ -671,11 +681,8 @@ trace_get_value_string(int t, char *buf, int len, float coeff[2], int i)
     chsnprintf(buf, len, "%.1f" 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');
-    }
+    v = groupdelay_from_array(i, array);
+    string_value_with_prefix(buf, len, v, 's');
     break;
   case TRC_LINEAR:
     v = linear(coeff);
@@ -844,7 +851,7 @@ void plot_into_index(float measured[2][101][2])
       if (!trace[t].enabled)
         continue;
       int n = trace[t].channel;
-      trace_index[t][i] = trace_into_index(x, t, i, measured[n][i]);
+      trace_index[t][i] = trace_into_index(x, t, i, measured[n]);
     }
   }
 #if 0
@@ -1520,7 +1527,7 @@ cell_draw_marker_info(int m, int n, int w, int h)
     xpos += strwidthpx + 4;
 
     slen = trace_get_info(t, buf, sizeof buf);
-    trace_get_value_string(t, buf+slen, (sizeof(buf))-slen, measured[trace[t].channel][idx], idx);
+    trace_get_value_string(t, buf+slen, (sizeof(buf))-slen, measured[trace[t].channel], idx);
     cell_drawstring_8x8_var(w, h, buf, xpos, ypos, config.trace_color[t], FALSE);
 
     j++;