[CORD-2719] Consider previous position of existing nodes in the graph

Change-Id: I1c814604391be3653d0515dfa3daf88cd780ed4a
diff --git a/src/app/service-graph/services/node-positioner.service.ts b/src/app/service-graph/services/node-positioner.service.ts
index 17823b7..bfe6c58 100644
--- a/src/app/service-graph/services/node-positioner.service.ts
+++ b/src/app/service-graph/services/node-positioner.service.ts
@@ -73,15 +73,15 @@
           return all;
         }, {});
 
-        // find the nodes that don't have a position defined and put them at the top
-        const allNodes = _.reduce(nodes, (all: string[], n: IXosSgNode) => {
+        // find the nodes that don't have a position defined and put them at the bottom
+        const allServiceNodes = _.reduce(nodes, (all: string[], n: IXosSgNode) => {
           if (n.type === 'service') {
             all.push(n.data.name);
           }
           return all;
         }, []);
         const positionedNodes = Object.keys(positionConstraints);
-        const unpositionedNodes = _.difference(allNodes, positionedNodes);
+        const unpositionedNodes = _.difference(allServiceNodes, positionedNodes);
 
         _.forEach(unpositionedNodes, (node: string, i: number) => {
           const hStep = this.getHorizontalStep(svg.width, unpositionedNodes);