Serial port data plotter in PyQt

I am rather new in GUI programming and multi-threading apps. It is just a serial port monitor which plot serial data via pyqtgraph. There are two curves on the first plot (a and b) and one curve on the second plot (c which is (a-a_prevous)/(b-b_prevous)). How can I improve this code in general? I use Python3.5.

from PyQt4 import QtGui
from PyQt4.QtCore import QObject, pyqtSignal
import sys
import serial
import pyqtgraph
import threading
import ui_main
import numpy as np

class GraphPlotter(QtGui.QMainWindow, ui_main.Ui_GraphPlotter):

    def __init__(self):
        super().__init__()
        pyqtgraph.setConfigOption('background', 'w')
        self.a = []
        self.b = []
        self.c = [0]
        self.flag = 'a'
        self.setupUi(self)
        self.plotAB.plotItem.showGrid(True, True, 0.7)
        self.plotC.plotItem.showGrid(True, True, 0.7)
        self.monitor = SerialMonitor()
        self.monitor.bufferUpdated.connect(self.update)
        self.startButton.clicked.connect(self.monitor.start)
        self.stopButton.clicked.connect(self.monitor.stop)
        self.clearBufferButton.clicked.connect(self.clear)

    def update(self, msg):
        if self.flag == 'a':
            self.a.append(msg)
            self.flag = 'b'

        elif self.flag == 'b':
            self.b.append(msg)
            c = pyqtgraph.hsvColor(0.2, alpha=.5)
            pen2 = pyqtgraph.mkPen(color=c, width=3)
            try:
                print((self.a[-1] - self.a[-2]), (self.b[-1] - self.b[-2]))
                self.c.append((self.a[-1] - self.a[-2]) / (self.b[-1] - self.b[-2]))
                self.plotC.plot(np.arange(len(self.c)), self.c, pen=pen2, clear=True)
            except ZeroDivisionError:
                print('Division by zero')
                self.c.append(0)
            except IndexError:
                print('C is not ready')
            finally:
                self.flag = 'a'

            for y, pen in [(self.a, (255, 0, 0)), (self.b, (0, 255, 0))]:
                self.plotAB.plot(np.arange(len(y)), y, pen=pen)

    def clear(self):
        self.a = []
        self.b = []
        self.c = [0]

        self.plotAB.clear()
        self.plotC.clear()


class SerialMonitor(QObject):
    bufferUpdated = pyqtSignal(int)

    def __init__(self):
        super(SerialMonitor, self).__init__()
        self.running = False
        self.thread = threading.Thread(target=self.serial_monitor_thread)

    def start(self):
        self.running = True
        self.thread.start()

    def stop(self):
        self.running = False

    def serial_monitor_thread(self):
        while self.running is True:
            ser = serial.Serial('/dev/ttyS0', 115200)
            msg = ser.readline()
            if msg:
                try:
                    self.bufferUpdated.emit(int(msg))
                except ValueError:
                    print('Wrong data')
            else:
                pass
            ser.close()


if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    plot = GraphPlotter()
    plot.show()
    app.exec_()