diff --git a/src/gui_builder.cc b/src/gui_builder.cc
index b583ad0..21e2fe6 100644
--- a/src/gui_builder.cc
+++ b/src/gui_builder.cc
@@ -262,7 +262,7 @@ GuiBuilder::GuiBuilder(const std::string& gladefile)
   // Main simulation widget  
   ScrolledWindow* simulation_window = NULL;
   _refXml->get_widget("SimulationScrolledWindow", simulation_window);
-  _simulation_widget = new SimulationWidget();
+  _simulation_widget = new SimulationWidget(Simulation::get_instance());
   SimulationWidget* simulation_widget = manage(_simulation_widget);
   simulation_window->add(*simulation_widget);
   simulation_widget->show();
diff --git a/src/simulation_widget.cc b/src/simulation_widget.cc
index a5affd9..791b979 100644
--- a/src/simulation_widget.cc
+++ b/src/simulation_widget.cc
@@ -34,33 +34,49 @@
 using namespace sgpem;
 
 
-SimulationWidget::SimulationWidget()
-    : Glib::ObjectBase("sgpem_SimulationWidget"), CairoWidget(),
-    SimulationObserver(), _simulation(0),
+SimulationWidget::SimulationWidget(Simulation& simulation)
+    : Glib::ObjectBase("sgpem_SimulationWidget"),
+    CairoWidget(),
+    SimulationObserver(),
+    HistoryObserver(),
+    _simulation(&simulation),
     _x_unit(10), _y_unit(10)
 
 {
-  // Register this observer:
-  Simulation::get_instance().attach(*this);
+  // Register this SimulationObserver:
+  _simulation->attach(*this);
 
+  // Register this HistoryObserver:
+  _simulation->get_history().attach(*this);
 }
 
 
 SimulationWidget::~SimulationWidget()
 {
-  Simulation::get_instance().detach(*this);
+  // Unregister this HistoryObserver:
+  _simulation->get_history().detach(*this);
+  
+  // Unregister this SimulationObserver:
+  _simulation->detach(*this);
 }
 
+#pragma argsused
 void
 SimulationWidget::update(const Simulation& changed_simulation)
 {
-  _simulation = &changed_simulation;
-
   // Force redraw
   //redraw();
   resize_redraw();
 }
 
+#pragma argsused
+void
+SimulationWidget::update(const History& changed_history)
+{
+  // Force redraw
+  //redraw();
+  resize_redraw();
+}
 
 void
 SimulationWidget::draw_widget(cairo_t* ctx)
@@ -73,7 +89,6 @@ SimulationWidget::draw_widget(cairo_t* ctx)
   
   */
   
-  const  Simulation& simu = Simulation::get_instance();
   double top_margin = _y_unit;
   double left_margin = _x_unit;
   double top_graph_margin    =  1.0 * _y_unit; //3.0 * _y_unit;
@@ -82,8 +97,8 @@ SimulationWidget::draw_widget(cairo_t* ctx)
   double process_bar_delta   =  1.0 * _y_unit;
   double process_bar_height  =  1.0 * _y_unit;
   double process_height = process_bar_height + 2*process_bar_delta;
-  Simulation::state sim_state = simu.get_state();
-  const  History& hist = simu.get_history();
+  Simulation::state sim_state = _simulation->get_state();
+  const  History& hist = _simulation->get_history();
   const  Environment::Processes& processes = hist.get_last_environment().get_processes();
   int    nproc = processes.size();
   double text_maxw = 0;
diff --git a/src/simulation_widget.hh b/src/simulation_widget.hh
index 22ab95b..4e26b0c 100644
--- a/src/simulation_widget.hh
+++ b/src/simulation_widget.hh
@@ -24,16 +24,18 @@
 #include "config.h"
 
 #include "cairo_widget.hh"
+#include "backend/history_observer.hh"
 #include "backend/simulation_observer.hh"
 
 namespace sgpem
 {
-  class SimulationWidget : public SimulationObserver, public  CairoWidget
+  class SimulationWidget : public SimulationObserver, public HistoryObserver, public  CairoWidget
   {
   public:
-    SimulationWidget();
+    SimulationWidget(Simulation& simulation);
     virtual ~SimulationWidget();
     void update(const Simulation& changed_simulation);
+    void update(const History& changed_history);
 
   protected:
     //virtual bool on_button_press_event(GdkEventButton* event);
@@ -46,7 +48,7 @@ namespace sgpem
   private:
     int _x_unit;
     int _y_unit;
-    const Simulation* _simulation;
+    Simulation* _simulation;
   };
 
 } //~ namespace sgpem
diff --git a/src/testsuite/test-simulation_widget.cc b/src/testsuite/test-simulation_widget.cc
index 6e8f0e6..34be9ac 100644
--- a/src/testsuite/test-simulation_widget.cc
+++ b/src/testsuite/test-simulation_widget.cc
@@ -405,7 +405,7 @@ TestWidget::change_scaling_mode()
 class MainWindow : public Gtk::Window
 {
 public:
-  MainWindow();
+  MainWindow(Simulation& simulation);
   virtual ~MainWindow();
 
 protected:
@@ -416,7 +416,7 @@ protected:
   virtual bool on_timer_timeout();
 
   // TestWidget  _test_widget;
-  SimulationWidget  _simulation_widget;
+  SimulationWidget _simulation_widget;
   Gtk::Button _start_button;
   Gtk::Button _stop_button;
   Gtk::Button _pause_button;
@@ -428,10 +428,14 @@ protected:
 };
 
 
-MainWindow::MainWindow() : _start_button("Start"), _stop_button("Stop"),
-  _pause_button("Pause"),  _runmode_button("Continue")
+MainWindow::MainWindow(Simulation& simulation)
+: _start_button("Start"),
+  _stop_button("Stop"),
+  _pause_button("Pause"),
+  _runmode_button("Continue"),
+  _simulation_widget(simulation),
+  _sim_state(Simulation::state_stopped)
 {
-  _sim_state = Simulation::state_stopped;
   // This just sets the title of our new window.
   set_title("Simulation Widget Test");
   add(_main_box);
@@ -608,7 +612,7 @@ main(int argc, char** argv)
   info << "END environment dump \n";
   info << "simulation state: " << get_simulation_state_name(simu.get_state()) << endl;
 
-  MainWindow win;
+  MainWindow win(simu);
 
   win.set_border_width(10);
   win.resize (400, 200);
@@ -671,7 +675,25 @@ void fillHistory(History &hist)
   Request&   req1 =  hist.add_request(p1_t1, 3);
 
   // add a sub request - Request, resource_key, duration, places
-  SubRequest& req1_sub1 = hist.add_subrequest(req1, respair.first, 5);
+  SubRequest& req1_sub1 = hist.add_subrequest(req1, respair.first, 6);
+  
+  // add a request - Thread, time
+  Request&   req2 =  hist.add_request(p2_t1, 1);
+
+  // add a sub request - Request, resource_key, duration, places
+  SubRequest& req2_sub1 = hist.add_subrequest(req2, respair.first, 4);
+
+  // add a request - Thread, time
+  Request&   req3 =  hist.add_request(p3_t1, 0);
+
+  // add a sub request - Request, resource_key, duration, places
+  SubRequest& req3_sub1 = hist.add_subrequest(req3, respair.first, 5);
+
+  // add a request - Thread, time
+  Request&   req3bis =  hist.add_request(p3_t1, 7);
+
+  // add a sub request - Request, resource_key, duration, places
+  SubRequest& req3bis_sub1 = hist.add_subrequest(req3bis, respair.first, 5);
 }