further simplify connected error graph

doc/src/error_connected.json

@@ -7,9 +7,6 @@
       { "name": "8" },
       { "name": "9" }
     ],
-    [
-      { "name": "11" }
-    ],
     [
       { "name": "16" },
       { "name": "17" },
@@ -32,14 +29,10 @@
     },
     {
       "source": "8",
-      "target": "11"
-    },
-    {
-      "source": "11",
       "target": "16"
     },
     {
-      "source": "11",
+      "source": "8",
       "target": "18"
     },
     {

error_after_combine2.json

@@ -19,13 +19,6 @@
         "name": "9"
       }
     ],
-    [
-      {
-        "layers": [],
-        "edges": [],
-        "name": "11"
-      }
-    ],
     [
       {
         "layers": [],
@@ -72,14 +65,10 @@
     },
     {
       "source": "8",
-      "target": "11"
-    },
-    {
-      "source": "11",
       "target": "16"
     },
     {
-      "source": "11",
+      "source": "8",
       "target": "18"
     },
     {

error_after_combine3.json

@@ -34,13 +34,6 @@
         "name": "6"
       }
     ],
-    [
-      {
-        "layers": [],
-        "edges": [],
-        "name": "7"
-      }
-    ],
     [
       {
         "layers": [],
@@ -84,14 +77,10 @@
     },
     {
       "source": "3",
-      "target": "7"
+      "target": "8"
     },
     {
       "source": "5",
-      "target": "7"
-    },
-    {
-      "source": "7",
       "target": "8"
     },
     {