"AssertionError: Expected 'show' to have been called once" when trying to mock plt.show()

Question

I am currently trying to design some tests to run with pytest for a GUI. The GUI has a button that plots a graph on click. I want to make sure the button does indeed generate a graph. At this stage, I am not verifying if the graph has the correct data; I am just interested in knowing if the graph does plot something.

My thinking is to check if a mock plt.show() is called whenever I call upon the method() of GraphButton: the class I use to generate the GUI and the graph. I use a mock so that the code does not load an actual graph, as that would require to manually close the window.

Unfortunately, when I run the file with pytest, I get the following message:

    def assert_called_once(self):
        """assert that the mock was called only once.
        """
        if not self.call_count == 1:
            msg = ("Expected '%s' to have been called once. Called %s times.%s"
                   % (self._mock_name or 'mock',
                      self.call_count,
                      self._calls_repr()))
>           raise AssertionError(msg)
E           AssertionError: Expected 'show' to have been called once. Called 0 times.

I have seen this question, unfortunately, the accepted answer gives me the same issue.

Here is the simplified code I used to design the test.

In the random_file:

import tkinter as tk
from tk import *
import matplotlib.pyplot as plt


class GraphButton:
    def __init__(self, window):
        self.window = window

    def graph(self):
        # Generate x values
        x = range(-10, 11)

        # Calculate y values (y = x)
        y = [val for val in x]

        # Plot the graph
        plt.plot(x, y)

        # Add labels and title
        plt.xlabel('x')
        plt.ylabel('y')
        plt.title('Graph of y = x')

        # Display the graph
        plt.grid(True)
        plt.show()

    def create_button(self):
        self.button = tk.Button(self.window, text="hello", command=self.graph)
        self.button.grid(column=0, row=0)

In the gui_test:

from random_file import GraphButton
import tkinter as tk
from tk import *
from unittest.mock import patch
import matplotlib


window = tk.Tk()
window.title("test gui")
window.geometry('500x500')

@patch('matplotlib.pyplot.show')
def test_button(mock_pyplot):
    base = GraphButton(window=window)
    base.create_button()
    base.button.invoke()
    mock_pyplot.show.assert_called_once()

Do you know why this happens?