Avoid graph extending outside the drawing area.

runtime/observatory/lib/src/elements/memory/graph.dart

Index: runtime/observatory/lib/src/elements/memory/graph.dart
diff --git a/runtime/observatory/lib/src/elements/memory/graph.dart b/runtime/observatory/lib/src/elements/memory/graph.dart
index 298527cdb4a060812cd2b45d864bcd299a09b422..a89ba43edf070b9f1ed8d2dc9d239294b62ffa9c 100644
--- a/runtime/observatory/lib/src/elements/memory/graph.dart
+++ b/runtime/observatory/lib/src/elements/memory/graph.dart
@@ -194,7 +194,7 @@ class MemoryGraphElement extends HtmlElement implements Renderable {
     if (_running) return;
     _running = true;
     final now = new DateTime.now();
-    final start = now.subtract(_window).subtract(_period);
+    final start = now.subtract(_window);
     // The Service classes order isolates from the older to the newer
     final isolates =
         (await Future.wait(_vm.isolates.map(_isolates.get))).reversed.toList();
@@ -205,6 +205,13 @@ class MemoryGraphElement extends HtmlElement implements Renderable {
       _isolateFreeSamples.removeAt(0);
     }
 
+    if (_ts.first.isAfter(start)) {
+      _ts.insert(0, start);
+      _vmSamples.insert(0, _vmSamples.first);
+      _isolateUsedSamples.insert(0, _isolateUsedSamples.first);
+      _isolateFreeSamples.insert(0, _isolateFreeSamples.first);
+    }
+
     if (_isolateIndex.length == 0) {
       _selected = isolates.length * 2;
       _onIsolateSelected.add(new IsolateSelectedEvent(isolates.last));