Affinity migration checkout

Issue #6

LayoutNode.js

@@ -8,6 +8,7 @@ class LayoutNode {
     this.affinity_ = [];
 
     this.label = this.graphNode_.label;
+    this.pageRank = this.graphNode_.pageRank;
     this.subgraph = this.graphNode_.subgraph;
 
     this.nodesByPos_.set(this.pos, this);
@@ -46,6 +47,26 @@ class LayoutNode {
         this.addAffinity(node, d => d <= 2 ? -INF : 0);
       }
 
+      // Am I in any labeled groups that node is not?
+      // If so, preserve one space above the group
+      let labeled = new Set(Array.from(this.groups).filter(g => !!g.label));
+      if (asymDifference(labeled, node.groups).size) {
+        node.addAffinity(this, (d, v) =>
+                         (v[0] == 0 && v[1] > 0 && v[1] < 2) ? -INF : 0);
+      }
+
+      // Try to stack nodes with the same label
+      if (node.label == this.label) {
+        this.addAffinity(node, (d, v) => v[0] == 0 ? 200 : 500);
+      }
+
+      // Try to preserve pagerank left-to-right flow from initial positions
+      let rankSign = Math.sign(node.pageRank - this.pageRank);
+      if (rankSign != 0) {
+        this.addAffinity(node, (d, v) =>
+                         [Math.sign(v[0]) == rankSign ? 0 : -1000, 0]);
+      }
+
       for (let group of this.groups) {
         // Ensure groups do not overlap
         if (group.nodes.has(node)) {