Python executable hangs with plt.show() - python-2.7

Question

I am creating a GUI application (EXE) in Python 2.7 on Win 10 platform.
The application is supposed to plot a graph (using matplotlib) with user provided parameters. When the user changes the values and presses the plot button again, new graph should be plotted on the same figure retaining the previous curves for ease of comparison (also a requirement).
I had to use plt.show() at the end of the script to show the graph when running the exe file of my program.
Problem: Application freezes and crashes when the plot button is pressed the second time. For the first time, everything goes fine and the graph is displayed.
Solutions tried:
Going thru so many threads, I tried following solutions without luck:
Used plt.draw() instead of plt.show() because the later blocks program execution. But draw() does not display the graphs.
Tried plt.pause(), the application gives RunTime error and crashes.
Also played with ion() / ioff().
Tried plt.show(block = False), the application crashes on second click on plot button.
I came across various backends that can be used with matplotlib and suspected that plt.show() might work well with other backends. Tried TkAgg, WxAgg, and others but no luck.
I would really appreciate if someone can help me with this...already tried for 3 days!!
Code sample:
from PyQt4 import QtCore, QtGui
import numpy as np
import matplotlib
matplotlib.use('TkAgg') #Trying different backends
import matplotlib.pyplot as plt
plt.rc('xtick', labelsize = 16)
plt.rc('ytick', labelsize = 16)
import FileDialog #These three imports needed for EXE
import tkFileDialog
import tkMessageBox
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
#UI elements code...
def retranslateUi(self, MainWindow):
#Other code lines
def my_Plot_Function():
#Some calculations
plt.figure('My title')
plt.plot(x, y, label = 'My Label')
plt.grid(True)
plt.axhline(y = 0, color = 'k')
plt.xlabel(' X ', fontsize = 16)
plt.ylabel(' Y ', fontsize = 16)
plt.legend(prop={'size': 14})
plt.show()
if __name__ == "__main__":
import sys
app = QtGui.QApplication(sys.argv)
MainWindow = QtGui.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())