client/graph: fix px overwriting x via d3. fixes #676
client/graph: fix px overwriting x via d3. fixes #676

Debugging helpers:

+    function assert_graph_has_xy() {
+        graph.all_nodes().forEach(function (n) {
+            util.assert(n.px !== undefined && !_.isNaN(n.px));
+            util.assert(n.py !== undefined && !_.isNaN(n.py));
+            util.assert(n.x !== undefined && !_.isNaN(n.x));
+            util.assert(n.y !== undefined && !_.isNaN(n.y));
+        });
+    }

And call it before calling alpha, forcing d3.tick call, which
copies px to x, and if px is undefined at that point it will result in
attempts to render using undefined positions.

Rhizi/rhizi,Rhizi/rhizi,Rhizi/rhizi,Rhizi/rhizi