pyqt signal not updating matplotlib FigureCanvasQTAgg and delayed

Question

I am building a GUI with PyQt4. At the moment I have a OpenGLWidget() that displays geometry at every N-th time step from Solver() that is in a different thread. I added new graph window - GraphWidget() to plot "some interesting" data. In Solver() I use a signal to refresh Open GL widget - self.updateGL(). I have connected the existing signal to also redraw FigureCanvasQTAgg but nothing is drawn. Here is the class SimulationControlWidget:

class SimulationControlWidget(self):
    self.OpenGLWidget = OpenGLWidget() #opengl widget
    self.GraphWidget = GraphWidget() #plot widget with matplotib

    # solver in new thread
    self.solver = Solver()
    self._solver_thread = QThread()
    self._solver_thread.start()
    self.solver.moveToThread(self._solver_thread)

    # signals
    #this signal is OK
    OKself.solver.solveODE.repaintGL_signal.signal_repaintGL.connect(self.OpenGLWidget.updateGL) 

    #this signal is not OK
    self.solver.solveODE.repaintGL_signal.signal_repaintGL.connect(self.GraphWidget.plot_graph)

Here is the class GraphWidget:

class GraphWidget(QtGui.QWidget):
    def __init__(self, MBD_system=[], parent=None, flags=0):
        super(GraphWidget, self).__init__(parent)
        self.fig = Figure((8.0, 4.0), dpi=100)
        self.canvas = FigureCanvas(self.fig)
        self.axes = self.fig.add_subplot(111)
        #   added button to manually execute function plot_graph()
        self.plot_button = QtGui.QPushButton("&Show")

        self.layout = QtGui.QVBoxLayout(self)
        self.layout.addWidget(self.canvas)
        self.layout.addWidget(self.plot_button)
        self.canvas.draw()

        #   signal to plot data with button self.plot_button
        self.plot_button.clicked.connect(self.plot_graph)

    def plot_graph(self):
        print 'plotting graph!'
        self.axes.cla()
        self.axes.plot(np.random.rand(10, 2))

        self.canvas.draw()

The problem I have is: if I manually click on the button self.plot_button the function plot_graph() is executed (random data is plotted correctly and text plotting_graph! is printed in eclipse pydev console). But if I try to execute function plot_graph() with signal only text plotting_graph! is printed in console and no data is plotted. I have also noticed if I use signal, the function plot_graph() is executing (due to signal) after the simulation has stopped. What should be the solution? I can also put the link to the entire code if it will be more useful. I think I will have to synchronize this...but how?