code: def test_encode_plot_to_base64(self):
      with utils.MatplotlibBackendMananger():
          import matplotlib.pyplot as plt
  
          plt.plot([1, 2, 3, 4])
          output_base64 = processing_utils.encode_plot_to_base64(plt)
      assert output_base64.startswith(
          "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAo"
      )
  
imports: import os
  import shutil
  import tempfile
  from copy import deepcopy
  from pathlib import Path
  from unittest.mock import patch
  import ffmpy
  import numpy
  import pytest
  from gradio_client import media_data
  from PIL import Image, ImageCms
  from gradio import processing_utils, utils
label_desc: Use image node_id 1 for calling the TestImagePreprocessing obj's underlying member method code with example usage: obj.test_encode_plot_to_base64() without return types