Matplotlib clearing the figure/axis for new plot - matplotlib

Question

am trying to figure out how to clear the axis in readiness for new plotting, I have tried ax.clf(), fig.clf() but nothing is happening. where am I not doing well? at the moment am not getting any errors and am using Matplotlib vers. 3.4.3.
from tkinter import *
import matplotlib.pyplot as plt
import numpy as np
import time
import datetime
import mysql.connector
import matplotlib.dates as mdates
my_connect = mysql.connector.connect(host="localhost", user="Kennedy", passwd="Kennerdol05071994", database="ecg_db", auth_plugin="mysql_native_password")
mycursor = my_connect.cursor()
voltage_container = []
time_container = []
def analyze_voltage_time():
global ax, fig
pat_id = 1
query = "SELECT voltage, time FROM ecg_data_tbl where patient_id = " +str(pat_id)
mycursor.execute(query)
result = mycursor .fetchall()
voltage, time = list(zip(*result))
for volts in voltage:
voltage_container.append(volts)
for tim in time:
time_container.append(str(tim))
fig = plt.figure(1, figsize = (15, 6), dpi = 80, constrained_layout = True)
ax = fig.add_subplot()
ax.plot(time_container, voltage_container)
for label in ax.get_xticklabels():
label.set_rotation(40)
label.set_horizontalalignment('right')
ax.set_title("Electrocadiogram")
ax.set_xlabel("Time(hh:mm:ss)")
ax.set_ylabel("Voltage(mV)")
ax.grid(b=True, which='major', color='#666666', linestyle='-')
ax.minorticks_on()
ax.grid(b=True, which='minor', color='#666666', linestyle='-', alpha=0.2)
plt.show()
def clear_():
ax.cla()
fig.clf()
# =================================MAIN GUI WINDOW======================================
analysis_window = Tk()
analysis_window.configure(background='light blue')
analysis_window.iconbitmap('lardmon_icon.ico')
analysis_window.title("ECG-LArdmon - ANALYZER")
analysis_window.geometry('400x200')
analysis_window.resizable(width=False, height=False)
# ===========================BUTTONS===================================
analyse_btn = Button(analysis_window, text='analyze', width = 20, command=analyze_voltage_time)
analyse_btn.pack()
clear_btn = Button(analysis_window, text= 'clear', width = 20, command=clear_)
clear_btn.pack()
analysis_window.mainloop()