diff --git a/Playgrounds/NumswPlayground.swift b/Playgrounds/NumswPlayground.swift
index 44d84f3..c6f0f71 100644
--- a/Playgrounds/NumswPlayground.swift
+++ b/Playgrounds/NumswPlayground.swift
@@ -13,29 +13,33 @@ import CoreGraphics
 #endif
 
 public class NumswPlayground {
+    
     internal init() {
         #if os(iOS)
-            viewController = RenderTableViewController()
+            viewController = RenderTableViewController(state: viewState)
         #endif
     }
     
     #if os(iOS)
         public let viewController: RenderTableViewController
+        internal let viewState = RenderTableViewController.State()
     #endif
     
-    public func append(renderer: ChartRenderer) {
-        renderers.append(renderer)
+    public func append(renderer: Renderer) {
+        #if os(iOS)
+        viewController.append(renderer: renderer)
+        #endif
     }
     
 #if os(iOS)
     public func print(_ matrix: Matrix<Double>) {
         let matrixTextRenderer = MatrixTextRenderer(matrix)
-        renderers.append(matrixTextRenderer)
+        append(renderer: matrixTextRenderer)
     }
     
     public func print(_ string: String) {
         let textRenderer = TextRenderer(string)
-        renderers.append(textRenderer)
+        append(renderer: textRenderer)
     }
 #endif
     
@@ -85,13 +89,14 @@ public class NumswPlayground {
         return _shared!
     }
     
-    private var renderers: [Renderer] = [] {
+    //private var renderers: [Renderer] = []
+    /*private var renderers: [ChartRenderer] = [] {
         didSet {
             #if os(iOS)
-            viewController.renderers = renderers.map { $0 as Renderer }
+            viewController.replace(renderers: renderers.map { $0 as Renderer })
             #endif
         }
-    }
+    }*/
     
     private var chartBuilder: ChartBuilder?
     
diff --git a/Playgrounds/PlaygroundKeyValueStore.swift b/Playgrounds/PlaygroundKeyValueStore.swift
new file mode 100644
index 0000000..ebc7723
--- /dev/null
+++ b/Playgrounds/PlaygroundKeyValueStore.swift
@@ -0,0 +1,68 @@
+//
+//  PlaygroundKeyValueStore.swift
+//  sandbox
+//
+//  Created by Yusuke Ito on 4/16/17.
+//  Copyright © 2017 sonson. All rights reserved.
+//
+
+
+// PlaygroundKeyValueStore as UserDefautls for sandbox app
+
+#if os(iOS)
+import Foundation
+
+
+#if SANDBOX_APP
+    //  sandbox app
+    internal enum PlaygroundValue {
+        case array([PlaygroundValue])
+        case boolean(Bool)
+        case data(Data)
+        case date(Date)
+        case dictionary([String: PlaygroundValue])
+        case floatingPoint(Double)
+        case integer(Int)
+        case string(String)
+    }
+    internal class PlaygroundKeyValueStore {
+        static let current = PlaygroundKeyValueStore()
+        private init() {
+            
+        }
+        subscript(key: String) -> PlaygroundValue? {
+            set {
+                print("KeyValueStore setting \(newValue as Any) for \(key)")
+                if let value = newValue {
+                    switch value {
+                    case .floatingPoint(let double):
+                        UserDefaults.standard.set(double, forKey: key)
+                    case .integer(let integer):
+                        UserDefaults.standard.set(integer, forKey: key)
+                    default:
+                        fatalError("Not implemented")
+                    }
+                } else {
+                    UserDefaults.standard.removeObject(forKey: key)
+                }
+            }
+            get {
+                print("KeyValueStore getting for \(key)")
+                guard let object = UserDefaults.standard.object(forKey: key) else {
+                    return nil
+                }
+                if let integer = object as? Int {
+                    return .integer(integer)
+                } else if let double = object as? Double {
+                    return .floatingPoint(double)
+                } else {
+                    fatalError("Not implemented")
+                }
+            }
+        }
+    }
+#else
+    //  playground
+#endif
+
+#endif
diff --git a/Playgrounds/RenderTableViewCell.swift b/Playgrounds/RenderTableViewCell.swift
index dffec98..43322ed 100644
--- a/Playgrounds/RenderTableViewCell.swift
+++ b/Playgrounds/RenderTableViewCell.swift
@@ -10,6 +10,8 @@
 import UIKit
 
 internal class RenderTableViewCell: UITableViewCell {
+        
+    private static let renderQueue = DispatchQueue(label: "Renderer-Queue")
     
     var renderer: Renderer? {
         willSet {
@@ -22,7 +24,7 @@ internal class RenderTableViewCell: UITableViewCell {
     
     override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
         super.init(style: .default, reuseIdentifier: reuseIdentifier)
-        print("RenderTableViewCell init")
+        //print("RenderTableViewCell init")
         self.separatorInset = .zero
         self.selectionStyle = .none
         renderImageView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
@@ -36,14 +38,18 @@ internal class RenderTableViewCell: UITableViewCell {
     }
     
     deinit {
-        print("RenderTableViewCell deinit")
+        //print("RenderTableViewCell deinit")
     }
     
     private var renderedImageSize = CGSize.zero
     
+    private var isImageViewDirty: Bool {
+        return renderedImageSize != self.contentView.bounds.size
+    }
+    
     func updateImageViewIfNeeded() {
         print("rendering bounds: \(self.contentView.bounds)")
-        if renderedImageSize == self.contentView.bounds.size &&
+        if isImageViewDirty == false &&
             self.renderImageView.image != nil {
             // already rendered
             return
@@ -53,9 +59,23 @@ internal class RenderTableViewCell: UITableViewCell {
     
     private func updateImageView() {
         guard let renderer = self.renderer else { return }
-        let image = renderer.renderToImage(size: self.contentView.bounds.size)
-        self.renderImageView.image = image
+        let withFadeAnimation = isImageViewDirty && self.renderImageView.image != nil
+        
         renderedImageSize = self.contentView.bounds.size
+        type(of: self).renderQueue.async {
+            let image = renderer.renderToImage(size: self.contentView.bounds.size)
+            DispatchQueue.main.async {
+                self.renderImageView.image = image
+                
+                if withFadeAnimation {
+                    let transition = CATransition()
+                    transition.duration = 0.25
+                    transition.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
+                    transition.type = kCATransitionFade
+                    self.renderImageView.layer.add(transition, forKey: nil)
+                }
+            }
+        }
     }
 }
 #endif
diff --git a/Playgrounds/RenderTableViewController.swift b/Playgrounds/RenderTableViewController.swift
index f1a00dc..522bd48 100755
--- a/Playgrounds/RenderTableViewController.swift
+++ b/Playgrounds/RenderTableViewController.swift
@@ -10,10 +10,18 @@
 
 #if os(iOS)
 import UIKit
+import QuartzCore
 
+#if SANDBOX_APP
+    //  sandbox app
+#else
+    //  playground
+    import PlaygroundSupport
+#endif
+    
 public class RenderTableViewController: UITableViewController, UZTextViewDelegate {
     
-    var renderers: [Renderer] = [] {
+    private var renderers: [Renderer] = [] {
         didSet {
             for i in 0..<renderers.count {
                 renderers[i].parentViewSize = self.tableView.frame.size
@@ -22,7 +30,41 @@ public class RenderTableViewController: UITableViewController, UZTextViewDelegat
         }
     }
     
-    public init() {
+    internal class State {
+        private enum Key: String {
+            case tableViewScrollOffsetX = "table_view_scrolloffset_x"
+            case tableViewScrollOffsetY = "table_view_scrolloffset_y"
+            case rendererCount = "renderer_count"
+        }
+        
+        var tableViewScrollOffset: CGPoint = .zero
+        init() {
+            let kvs = PlaygroundKeyValueStore.current
+            if let xValue = kvs[Key.tableViewScrollOffsetX.rawValue],
+                case .floatingPoint(let x) = xValue,
+                let yValue = kvs[Key.tableViewScrollOffsetY.rawValue],
+                case .floatingPoint(let y) = yValue {
+                tableViewScrollOffset = CGPoint(x: x, y: y)
+            } else {
+                tableViewScrollOffset = .zero
+            }
+        }
+        
+        func reset() {
+            tableViewScrollOffset = .zero
+        }
+        
+        func sync() {
+            let kvs = PlaygroundKeyValueStore.current
+            kvs[Key.tableViewScrollOffsetX.rawValue] = .floatingPoint(Double(tableViewScrollOffset.x))
+            kvs[Key.tableViewScrollOffsetY.rawValue] = .floatingPoint(Double(tableViewScrollOffset.y))
+        }
+    }
+    
+    private var state: State
+    
+    internal init(state: State) {
+        self.state = state
         super.init(style: .plain)
     }
     
@@ -41,20 +83,65 @@ public class RenderTableViewController: UITableViewController, UZTextViewDelegat
         tableView.register(TextTableViewCell.self, forCellReuseIdentifier: "TextTableViewCell")
     }
     
+    // not in use?
+    /*public func replace(renderers: [Renderer]) {
+        self.renderers = renderers
+        self.tableView.reloadData()
+    }*/
+    
+    public func removeAllRenderers() {
+        self.renderers = []
+        self.tableView.reloadData()
+    }
+    
     public func append(renderer: Renderer) {
+        // partial update
         self.renderers.append(renderer)
-        self.tableView.reloadData()
+        //self.tableView.insertRows(at: [IndexPath(row: renderers.count-1, section: 0)], with: .none)
+        tableView.reloadData()
+        
+        // scroll to previous content position
+        let lastRowOffset = tableView.rectForRow(at: IndexPath(row: renderers.count-1, section: 0))
+        
+        let viewBounds = UIEdgeInsetsInsetRect(view.bounds, tableView.contentInset)
+        let maxYOffset = max(lastRowOffset.origin.y + lastRowOffset.size.height - viewBounds.size.height, 0) // 0 ~
+        let offset = CGPoint(x: state.tableViewScrollOffset.x, y: min(maxYOffset, state.tableViewScrollOffset.y))
+        tableView.setContentOffset(offset, animated: false)
     }
 
+    // some times not invoked?? on iPad playground
     public override func viewDidLayoutSubviews() {
         super.viewDidLayoutSubviews()
-        // will ??
-        //self.tableView.reloadData()
+        updateVisibleCellImagesIfNeeded()
     }
     
+    // some times not invoked?? on iPad playground
     public override func viewWillLayoutSubviews() {
         super.viewWillLayoutSubviews()
-        tableView.visibleCells.flatMap({ $0 as? RenderTableViewCell}).forEach({ $0.updateImageViewIfNeeded() })
+        updateVisibleCellImagesIfNeeded()
+    }
+    
+    private func updateVisibleCellImagesIfNeeded() {
+        // on orientation changed
+        for view in tableView.visibleCells {
+            (view as? RenderTableViewCell)?.updateImageViewIfNeeded()
+        }
+    }
+    
+    // some times not invoked?? on iPad playground
+    /*public override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
+        updateVisibleCellImagesIfNeeded()
+        
+        super.viewWillTransition(to: size, with: coordinator)
+    }*/
+    
+    // TODO:
+    // `view*LayoutSubviews` seems to be not invoked on iPad playground
+    // temporary, we use this deprecated method until the solution is found
+    public override func willAnimateRotation(to toInterfaceOrientation: UIInterfaceOrientation, duration: TimeInterval) {
+        super.willAnimateRotation(to: toInterfaceOrientation, duration: duration)
+        state.reset()
+        updateVisibleCellImagesIfNeeded()
     }
 
     public override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
@@ -98,5 +185,30 @@ public class RenderTableViewController: UITableViewController, UZTextViewDelegat
     public func selectingStringEnded(_ textView: UZTextView) {
         self.tableView.isScrollEnabled = true
     }
+    
+    #if SANDBOX_APP
+    // Tap any table view cell to dismiss
+    public override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
+        self.navigationController?.popViewController(animated: true)
+    }
+    #endif
+    
+    public override func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
+        storeTableViewScrollOffset()
+    }
+    
+    public override func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
+        storeTableViewScrollOffset()
+    }
+    
+    public override func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
+        storeTableViewScrollOffset()
+    }
+    
+    private func storeTableViewScrollOffset() {
+        // store tableview scroll offset
+        state.tableViewScrollOffset = tableView.contentOffset
+        state.sync()
+    }
 }
 #endif
diff --git a/Playgrounds/sandbox/sandbox.xcodeproj/project.pbxproj b/Playgrounds/sandbox/sandbox.xcodeproj/project.pbxproj
index c6dd7eb..a9b0a24 100644
--- a/Playgrounds/sandbox/sandbox.xcodeproj/project.pbxproj
+++ b/Playgrounds/sandbox/sandbox.xcodeproj/project.pbxproj
@@ -19,6 +19,8 @@
 		148967C11E9786D900F19D91 /* TextTableViewCell.swift in Sources */ = {isa = PBXBuildFile; fileRef = 148967C01E9786D900F19D91 /* TextTableViewCell.swift */; };
 		148967C31E9786F400F19D91 /* RenderTableViewCell.swift in Sources */ = {isa = PBXBuildFile; fileRef = 148967C21E9786F400F19D91 /* RenderTableViewCell.swift */; };
 		148967C71E97921800F19D91 /* SelectTableViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = 148967C61E97921800F19D91 /* SelectTableViewController.swift */; };
+		52CCAD111EA3913500D6D6EB /* PlaygroundKeyValueStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 52CCAD101EA3913500D6D6EB /* PlaygroundKeyValueStore.swift */; };
+		52CCAD121EA3913500D6D6EB /* PlaygroundKeyValueStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 52CCAD101EA3913500D6D6EB /* PlaygroundKeyValueStore.swift */; };
 		D68EF7091E86C8C600927991 /* AxisRenderer.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A0B5BB1E6FE801004AE3B4 /* AxisRenderer.swift */; };
 		D68EF70A1E86C8C600927991 /* Chart.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A0B5BC1E6FE801004AE3B4 /* Chart.swift */; };
 		D68EF70B1E86C8C600927991 /* ChartBuilder.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A0B5BD1E6FE801004AE3B4 /* ChartBuilder.swift */; };
@@ -128,6 +130,7 @@
 		148967C01E9786D900F19D91 /* TextTableViewCell.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = TextTableViewCell.swift; path = ../TextTableViewCell.swift; sourceTree = "<group>"; };
 		148967C21E9786F400F19D91 /* RenderTableViewCell.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = RenderTableViewCell.swift; path = ../RenderTableViewCell.swift; sourceTree = "<group>"; };
 		148967C61E97921800F19D91 /* SelectTableViewController.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = SelectTableViewController.swift; sourceTree = "<group>"; };
+		52CCAD101EA3913500D6D6EB /* PlaygroundKeyValueStore.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; name = PlaygroundKeyValueStore.swift; path = ../PlaygroundKeyValueStore.swift; sourceTree = "<group>"; };
 		52E1EE3E1E6A8A2E008D19BB /* sandboxTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = sandboxTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		52E1EE421E6A8A2E008D19BB /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		D68EF6FD1E86C89600927991 /* NumswRenderer.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = NumswRenderer.framework; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -279,6 +282,7 @@
 				144A150B1E971A1400B42288 /* MatrixTextRenderer.swift */,
 				D6A0B5C41E6FE801004AE3B4 /* NumswPlayground_Playground.swift */,
 				D6A0B5C51E6FE801004AE3B4 /* NumswPlayground.swift */,
+				52CCAD101EA3913500D6D6EB /* PlaygroundKeyValueStore.swift */,
 				D6A0B5C61E6FE801004AE3B4 /* Renderer.swift */,
 				D6A0B5C91E6FE801004AE3B4 /* RendererUtil.swift */,
 				D6A0B5CA1E6FE801004AE3B4 /* RenderScrollViewController.swift */,
@@ -606,6 +610,7 @@
 				144A150C1E971A1400B42288 /* MatrixTextRenderer.swift in Sources */,
 				D68EF7101E86C8C600927991 /* LineGraph.swift in Sources */,
 				144A15101E971B9D00B42288 /* UZCursor.swift in Sources */,
+				52CCAD111EA3913500D6D6EB /* PlaygroundKeyValueStore.swift in Sources */,
 				D68EF7151E86C8C600927991 /* RendererUtil.swift in Sources */,
 				144A15131E971C0400B42288 /* UZLoupe.swift in Sources */,
 				144A15121E971B9D00B42288 /* UZTextView.swift in Sources */,
@@ -630,6 +635,7 @@
 				D68EF77B1E86DA4C00927991 /* ScatterGraphRenderer.swift in Sources */,
 				D68EF77C1E86DA4C00927991 /* ScatterGraph.swift in Sources */,
 				D68EF77D1E86DA4C00927991 /* Renderer.swift in Sources */,
+				52CCAD121EA3913500D6D6EB /* PlaygroundKeyValueStore.swift in Sources */,
 				D68EF77E1E86DA4C00927991 /* DummyData.swift in Sources */,
 				D68EF77F1E86DA4C00927991 /* RenderScrollViewController.swift in Sources */,
 				D68EF7801E86DA4C00927991 /* LineGraph.swift in Sources */,