Matplotlib Slider widget for discrete choice on tkinter

Question

I am applying the following discussion in my codes
Can I make matplotlib sliders more discrete?

The following codes is to plot contour from five (slider: 0 ~ 4) .xlsx files on tkinter. Each file just contains numerical data in the matrix 12X6 such as

1. I did not use ipywidgets.wIntSlider since for tkinter I want to use canvas = FigureCanvasTkAgg(fig, root) and ipywidgets.wIntSlider cannot work on 'fig'.
2. The problem comes from although the values are integer by using valfmt='%0.0f', the slider itself is so continuous such that it cannot pinpoint on the integer values.

---

from tkinter import *
import tkinter.ttk as ttk
import matplotlib
import matplotlib.pyplot as plt
import numpy as np
import os
import pandas as pd
%matplotlib widget
from matplotlib.widgets import Slider
from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg
matplotlib.use('TkAgg')

root = Tk()
root.title('TEST')
root.geometry("800x800")

def plot_noise():
    # ============================================Read .xlsx file=====================================
    folder = r'C:\Users\Denny\Desktop\Work\test_read'
    files = os.listdir(folder)
    dfs = {}
    for i, file in enumerate(files):
        if file.endswith('.xlsx'):
            dfs[i] = pd.read_excel(os.path.join(folder,file), sheet_name='Z=143', header = None, skiprows=[0], usecols = "B:M")

    num = i + 1
    rec = np.shape(dfs[0])
    # =================================================PLOT===========================================
    fig = plt.Figure()                       
    canvas = FigureCanvasTkAgg(fig, root)     
    canvas.get_tk_widget().grid(row=6, column=0, columnspan=3, rowspan=3, sticky=W+E+N+S, padx=0, pady=0) 
    
    # ===============================================contourf=========================================
    ax = fig.add_subplot(111)                 
    fig.subplots_adjust(bottom=0.25)          
    X = np.arange(1,rec[1]+1,1)
    Y = np.arange(1,rec[0]+1,1)
    x , y = np.meshgrid(X,Y)   
    con = ax.contourf(x,y,dfs[1])              
    ax.axis([1, 12, 1, 6])
    # ================================================Slider==========================================
    global slider_de
    slider_bar = fig.add_axes([0.12, 0.1, 0.78, 0.03])     
    slider_de = Slider(slider_bar, 's_bar', 0, num-1, valinit=1,valfmt='%0.0f')
    num_on_slider = []
    def update(val):
        num_on_slider.append(slider_de.val)
        for ii in range(0,num):
            if num_on_slider[-1] == ii:
                con = ax.contourf(x,y,dfs[ii])
                ax.axis([1, 12, 1, 6])
                
    slider_de.on_changed(update)            
                
    
# =================================================GUI - Tkinter======================================= 
resultButton = ttk.Button(root, text = 'show', command = plot_noise)
resultButton.grid(column=0, row=1, pady=15, sticky=W)

root.mainloop()

So when slider is at the initial values and two sides of frame (must be integer), the plot changes; however, at the other position, the plot will not change.

How to fix that? Thanks!!