Persist variable settings after graph is plotted (#1055)

* Allow copying graph as image

* Persist variables

* Revert "Allow copying graph as image"

This reverts commit 4fc9d798bc5f3ff82efc4fb00140103213fb81e2.

* fix binding bug

* undo cert change

* fix animation

* remove extra lines

* remove overrides

* undo key comment

src/GraphControl/Models/Variable.h

@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+#pragma once
+#include "Utils.h"
+
+namespace GraphControl
+{
+public
+    ref class Variable sealed
+    {
+    public:
+        PROPERTY_RW(double, Value);
+        PROPERTY_RW(double, Step);
+        PROPERTY_RW(double, Min);
+        PROPERTY_RW(double, Max);
+
+        Variable(double value)
+            : m_Value{ value }
+            , m_Step{ 0.1 }
+            , m_Min{ 0.0 }
+            , m_Max{ 2.0 }
+        {
+        }
+    };
+}