问Tkinter中的Embedded Matplotlib图只在按键事件上按下'g‘时用新数据进行更新。为什么？

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import tkinter
from random import randint
import matplotlib as plt
import numpy as np

from matplotlib.backends.backend_tkagg import (
    FigureCanvasTkAgg, NavigationToolbar2Tk)
# Implement the default Matplotlib key bindings.
from matplotlib.backend_bases import key_press_handler
from matplotlib.figure import Figure

root = tkinter.Tk()
root.wm_title("Embedding in Tk")

#create figure
fig = Figure(figsize=(5, 4), dpi=100)
ax = fig.add_axes([0,0,1,1])
ax.imshow(np.array([[0,10],[23,40]]))

#plt.rcParams["keymap.grid"] = ""

#create canvas with figure
canvas = FigureCanvasTkAgg(fig, master=root)  # A tk.DrawingArea.

canvas.get_tk_widget().pack(side=tkinter.TOP, fill=tkinter.BOTH, expand=1)

def on_key_press(event):
    #if event.key == 'x':
    ax.imshow(np.array([[randint(0,30),randint(0,30)],[randint(0,30),randint(0,30)]]))

    key_press_handler(event, canvas) # toolbar)
    print("you pressed {}".format(event.key))

#connect canvas to event function
canvas.mpl_connect("key_press_event", on_key_press)

def _quit():
    root.quit()     # stops mainloop
    root.destroy()  # this is necessary on Windows to prevent
                    # Fatal Python Error: PyEval_RestoreThread: NULL tstate

button = tkinter.Button(master=root, text="Quit", command=_quit)
button.pack(side=tkinter.BOTTOM)

tkinter.mainloop()