sets callback function for updating the plot.
        in the callback function implement the logic of reading of serial input
        also the further processing of the signal if necessary has to be done
        in this
        callbak function.

def set_call_back(self, func):

        """sets callback function for updating the plot.
        in the callback function implement the logic of reading of serial input
        also the further processing of the signal if necessary has to be done
        in this
        callbak function."""

        self.timer.add_callback(func)
        self.timer.start()